_if__(!_GENERIC__)
Copyright © 1988, 1989, 1990, 1991 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled “GNU General Public License” is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled “GNU General Public License” may be included in a translation approved by the Free Software Foundation instead of in the original English.
_if__(0)
THIS IS THE SOURCE PRIOR TO PREPROCESSING. The full source needs to be run through m4 before either tex- or info- formatting: for example, _0__ m4 pretex.m4 none.m4 all.m4 gdb.texinfo >gdb-all.texinfo _1__ will produce (assuming your path finds either GNU m4 >= 0.84, or SysV m4; Berkeley won’t do) a file suitable for formatting. See the text in "pretex.m4" for a fuller explanation (and the macro definitions).
_fi__(0) _include__(gdbVN.m4) _if__(_GENERIC__) _fi__(_GENERIC__) _if__(!_GENERIC__) _fi__(!_GENERIC__)
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The purpose of a debugger such as _GDBN__ is to allow you to see what is going on “inside” another program while it executes—or what another program was doing at the moment it crashed.
_GDBN__ can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:
You can use _GDBN__ to debug programs written in C, C++, and Modula-2. Fortran support will be added when a GNU Fortran compiler is ready.
Free Software | ||
Contributors to GDB |
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_GDBN__ is free software, protected by the GNU General Public License (GPL). The GPL gives you the freedom to copy or adapt a licensed program—but every person getting a copy also gets with it the freedom to modify that copy (which means that they must get access to the source code), and the freedom to distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve these freedoms.
Fundamentally, the General Public License is a license which says that you have these freedoms and that you can’t take these freedoms away from anyone else.
For full details, see section GNU GENERAL PUBLIC LICENSE.
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Richard Stallman was the original author of GDB, and of many other GNU programs. Many others have contributed to its development. This section attempts to credit major contributors. One of the virtues of free software is that everyone is free to contribute to it; with regret, we cannot actually acknowledge everyone here. The file ‘ChangeLog’ in the GDB distribution approximates a blow-by-blow account.
Changes much prior to version 2.0 are lost in the mists of time.
Plea: Additions to this section are particularly welcome. If you or your friends (or enemies; let’s be evenhanded) have been unfairly omitted from this list, we would like to add your names!
So that they may not regard their long labor as thankless, we particularly thank those who shepherded GDB through major releases: John Gilmore (releases _GDB_VN__, 4.1, 4.0); Jim Kingdon (releases 3.9, 3.5, 3.4, 3.3); and Randy Smith (releases 3.2, 3.1, 3.0). As major maintainer of GDB for some period, each contributed significantly to the structure, stability, and capabilities of the entire debugger.
Richard Stallman, assisted at various times by Pete TerMaat, Chris Hanson, and Richard Mlynarik, handled releases through 2.8.
Michael Tiemann is the author of most of the GNU C++ support in GDB, with significant additional contributions from Per Bothner. James Clark wrote the GNU C++ demangler. Early work on C++ was by Peter TerMaat (who also did much general update work leading to release 3.0).
GDB _GDB_VN__ uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project of V. Gumby Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.
David Johnson wrote the original COFF support; Pace Willison did the original support for encapsulated COFF.
Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS support. Jean-Daniel Fekete contributed Sun 386i support. Chris Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki Hasei contributed Sony/News OS 3 support. David Johnson contributed Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support. Keith Packard contributed NS32K support. Doug Rabson contributed Acorn Risc Machine support. Chris Smith contributed Convex support (and Fortran debugging). Jonathan Stone contributed Pyramid support. Michael Tiemann contributed SPARC support. Tim Tucker contributed support for the Gould NP1 and Gould Powernode. Pace Willison contributed Intel 386 support. Jay Vosburgh contributed Symmetry support.
Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.
Jay Fenlason and Roland McGrath ensured that GDB and GAS agree about several machine instruction sets.
Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote debugging. Intel Corporation and Wind River Systems contributed remote debugging modules for their products.
Brian Fox is the author of the readline libraries providing command-line editing and command history.
Andrew Beers of SUNY Buffalo wrote the language-switching code and the Modula-2 support, and contributed the Languages chapter of this manual.
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Using the new command target
, you can select at runtime whether
you are debugging local files, local processes, standalone systems over
a serial port, realtime systems over a TCP/IP connection, etc. The
command load
can download programs into a remote system. Serial
stubs are available for Motorola 680x0 and Intel 80386 remote systems;
_GDBN__ also supports debugging realtime processes running under
VxWorks, using SunRPC Remote Procedure Calls over TCP/IP to talk to a
debugger stub on the target system. Internally, _GDBN__ now uses a
function vector to mediate access to different targets; if you need to
add your own support for a remote protocol, this makes it much easier.
_GDBN__ now sports watchpoints as well as breakpoints. You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place in your program where this may happen.
Commands that issue wide output now insert newlines at places designed to make the output more readable.
_GDBN__ uses a new library called the Binary File Descriptor (BFD) Library to permit it to switch dynamically, without reconfiguration or recompilation, between different object-file formats. Formats currently supported are COFF, a.out, and the Intel 960 b.out; files may be read as .o’s, archive libraries, or core dumps. BFD is available as a subroutine library so that other programs may take advantage of it, and the other GNU binary utilities are being converted to use it.
Compile-time configuration (to select a particular architecture and
operating system) is much easier. The script configure
now
allows you to configure _GDBN__ as either a native debugger or a
cross-debugger. See section Installing _GDBN__ for details on how to
configure and on what architectures are now available.
The user interface to _GDBN__’s control variables has been simplified
and consolidated in two commands, set
and show
. Output
lines are now broken at readable places, rather than overflowing onto
the next line. You can suppress output of machine-level addresses,
displaying only source language information.
_GDBN__ now supports C++ multiple inheritance (if used with a GCC
version 2 compiler), and also has limited support for C++ exception
handling, with the commands catch
and info catch
: _GDBN__
can break when an exception is raised, before the stack is peeled back
to the exception handler’s context.
_GDBN__ now has preliminary support for the GNU Modula-2 compiler, currently under development at the State University of New York at Buffalo. Coordinated development of both _GDBN__ and the GNU Modula-2 compiler will continue through the fall of 1991 and into 1992. Other Modula-2 compilers are currently not supported, and attempting to debug programs compiled with them will likely result in an error as the symbol table of the executable is read in.
Many _GDBN__ commands have been renamed to make them easier to remember
and use. In particular, the subcommands of info
and
show
/set
are grouped to make the former refer to the state
of your program, and the latter refer to the state of _GDBN__ itself.
See section Renamed Commands, for details on what commands were renamed.
_GDBN__ _GDB_VN__ can debug programs and core files that use SunOS shared libraries.
_GDBN__ _GDB_VN__ has a reference card; See section Formatting the Documentation for instructions on printing it.
Kernel debugging for BSD and Mach systems; Tahoe and HPPA architecture support.
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You can use this manual at your leisure to read all about _GDBN__. However, a handful of commands are enough to get started using the debugger. This chapter illustrates these commands.
_0__
One of the preliminary versions of GNU m4
(a generic macro
processor) exhibits the following bug: sometimes, when we change its
quote strings from the default, the commands used to capture one macro’s
definition in another stop working. In the following short m4
session, we define a macro foo
which expands to 0000
; we
then use the m4
builtin defn
to define bar
as the
same thing. However, when we change the open quote string to
<QUOTE>
and the close quote string to <UNQUOTE>
, the same
procedure fails to define a new synonym baz
:
$ cd gnu/m4 $ ./m4 define(foo,0000) foo 0000 define(bar,defn(`foo')) bar 0000 changequote(<QUOTE>,<UNQUOTE>) define(baz,defn(<QUOTE>foo<UNQUOTE>)) baz C-d m4: End of input: 0: fatal error: EOF in string
Let’s use _GDBN__ to try to see what’s going on.
$ _GDBP__ m4 GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is absolutely no warranty for GDB; type "show warranty" for details. GDB _GDB_VN__, Copyright 1991 Free Software Foundation, Inc... (_GDBP__)
_GDBN__ reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly. We then tell _GDBN__ to use a narrower display width than usual, so that examples will fit in this manual.
(_GDBP__) set width 70
Let’s see how the m4
builtin changequote
works.
Having looked at the source, we know the relevant subroutine is
m4_changequote
, so we set a breakpoint there with _GDBN__’s
break
command.
(_GDBP__) break m4_changequote Breakpoint 1 at 0x62f4: file builtin.c, line 879.
Using the run
command, we start m4
running under _GDBN__
control; as long as control does not reach the m4_changequote
subroutine, the program runs as usual:
(_GDBP__) run Starting program: /work/Editorial/gdb/gnu/m4/m4 define(foo,0000) foo 0000
To trigger the breakpoint, we call changequote
. _GDBN__
suspends execution of m4
, displaying information about the
context where it stops.
changequote(<QUOTE>,<UNQUOTE>) Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) at builtin.c:879 879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]), argc, 1, 3))
Now we use the command n
(next
) to advance execution to
the next line of the current function.
(_GDBP__) n 882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1]) : nil,
set_quotes
looks like a promising subroutine. We can go into it
by using the command s
(step
) instead of next
.
step
goes to the next line to be executed in any
subroutine, so it steps into set_quotes
.
(_GDBP__) s set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") at input.c:530 530 if (lquote != def_lquote)
The summary display showing the subroutine where m4
is now
suspended (and its arguments) is called a stack frame display. We can
use the backtrace
command (which can also be spelled bt
),
to see where we are in the stack: it displays a stack frame for each
active subroutine.
(_GDBP__) bt #0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") at input.c:530 #1 0x6344 in m4_changequote (argc=3, argv=0x33c70) at builtin.c:882 #2 0x8174 in expand_macro (sym=0x33320) at macro.c:242 #3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30) at macro.c:71 #4 0x79dc in expand_input () at macro.c:40 #5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
Let’s step through a few more lines to see what happens. The first two
times, we can use ‘s’; the next two times we use n
to avoid
falling into the xstrdup
subroutine.
(_GDBP__) s 0x3b5c 532 if (rquote != def_rquote) (_GDBP__) s 0x3b80 535 lquote = (lq == nil || *lq == '\0') ? def_lquote :\ xstrdup(lq); (_GDBP__) n 536 rquote = (rq == nil || *rq == '\0') ? def_rquote : xstrdup\ (rq); (_GDBP__) n 538 len_lquote = strlen(rquote);
The last line displayed looks a little odd; let’s examine the variables
lquote
and rquote
to see if they are in fact the new left
and right quotes we specified. We can use the command p
(print
) to see their values.
(_GDBP__) p lquote $1 = 0x35d40 "<QUOTE>" (_GDBP__) p rquote $2 = 0x35d50 "<UNQUOTE>"
lquote
and rquote
are indeed the new left and right quotes.
Let’s look at some context; we can display ten lines of source
surrounding the current line, with the l
(list
) command.
(_GDBP__) l 533 xfree(rquote); 534 535 lquote = (lq == nil || *lq == '\0') ? def_lquote : xstrdup\ (lq); 536 rquote = (rq == nil || *rq == '\0') ? def_rquote : xstrdup\ (rq); 537 538 len_lquote = strlen(rquote); 539 len_rquote = strlen(lquote); 540 } 541 542 void
Let’s step past the two lines that set len_lquote
and
len_rquote
, and then examine the values of those variables.
(_GDBP__) n 539 len_rquote = strlen(lquote); (_GDBP__) n 540 } (_GDBP__) p len_lquote $3 = 9 (_GDBP__) p len_rquote $4 = 7
That certainly looks wrong, assuming len_lquote
and
len_rquote
are meant to be the lengths of lquote
and
rquote
respectively. Let’s try setting them to better values.
We can use the p
command for this, since it’ll print the value of
any expression—and that expression can include subroutine calls and
assignments.
(_GDBP__) p len_lquote=strlen(lquote) $5 = 7 (_GDBP__) p len_rquote=strlen(rquote) $6 = 9
Let’s see if that fixes the problem of using the new quotes with the
m4
built-in defn
. We can allow m4
to continue
executing with the c
(continue
) command, and then try the
example that caused trouble initially:
(_GDBP__) c Continuing. define(baz,defn(<QUOTE>foo<UNQUOTE>)) baz 0000
Success! The new quotes now work just as well as the default ones. The
problem seems to have been just the two typos defining the wrong
lengths. We’ll let m4
exit by giving it an EOF as input.
C-d Program exited normally.
The message ‘Program exited normally.’ is from _GDBN__; it
indicates m4
has finished executing. We can end our _GDBN__
session with the _GDBN__ quit
command.
(_GDBP__) quit _1__
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2.1 Starting _GDBN__ | ||
2.2 Leaving _GDBN__ | ||
2.3 Shell Commands |
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_GDBN__ is invoked with the shell command _GDBP__
. Once started,
it reads commands from the terminal until you tell it to exit.
You can run _GDBP__
with no arguments or options; but the most
usual way to start _GDBN__ is with one argument or two, specifying an
executable program as the argument:
_GDBP__ program
You can also start with both an executable program and a core file specified:
_GDBP__ program core
You can, instead, specify a process ID as a second argument, if you want to debug a running process:
_GDBP__ program 1234
would attach _GDBN__ to process 1234
(unless you also have a file
named ‘1234’; _GDBN__ does check for a core file first).
You can further control how _GDBN__ starts up by using command-line options. _GDBN__ itself can remind you of the options available:
_GDBP__ -help
will display all available options and briefly describe their use (‘_GDBP__ -h’ is a shorter equivalent).
All options and command line arguments you give are processed in sequential order. The order makes a difference when the ‘-x’ option is used.
2.1.1 Choosing Files | ||
2.1.2 Choosing Modes | Choosing Modes _if__(!_GENERIC__) _include__(gdbinv-m.m4)_dnl__ _fi__(!_GENERIC__) |
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As shown above, any arguments other than options specify an executable file and core file; that is, the first argument encountered with no associated option flag is equivalent to a ‘-se’ option, and the second, if any, is equivalent to a ‘-c’ option. Many options have both long and short forms; both are shown here. The long forms are also recognized if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with ‘+’ rather than ‘-’, though we illustrate the more usual convention.)
-symbols=file
-s file
Read symbol table from file file.
-exec=file
-e file
Use file file as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.
-se=file
Read symbol table from file file and use it as the executable file.
-core=file
-c file
Use file file as a core dump to examine.
-command=file
-x file
Execute _GDBN__ commands from file file. See section Command Files.
-directory=directory
-d directory
Add directory to the path to search for source files.
_if__(!_GENERIC__)
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_fi__(!_GENERIC__) _if__(_GENERIC__) _fi__(_GENERIC__)
-nx
-n
Do not execute commands from any ‘_GDBINIT__’ initialization files. Normally, the commands in these files are executed after all the command options and arguments have been processed. See section Command Files.
-quiet
-q
“Quiet”. Do not print the introductory and copyright messages. These messages are also suppressed in batch mode.
-batch
Run in batch mode. Exit with status 0
after processing all the command
files specified with ‘-x’ (and ‘_GDBINIT__’, if not inhibited).
Exit with nonzero status if an error occurs in executing the _GDBN__
commands in the command files.
Batch mode may be useful for running _GDBN__ as a filter, for example to download and run a program on another computer; in order to make this more useful, the message
Program exited normally.
(which is ordinarily issued whenever a program running under _GDBN__ control terminates) is not issued when running in batch mode.
-cd=directory
Run _GDBN__ using directory as its working directory, instead of the current directory.
-fullname
-f
Emacs sets this option when it runs _GDBN__ as a subprocess. It tells _GDBN__ to output the full file name and line number in a standard, recognizable fashion each time a stack frame is displayed (which includes each time the program stops). This recognizable format looks like two ‘\032’ characters, followed by the file name, line number and character position separated by colons, and a newline. The Emacs-to-_GDBN__ interface program uses the two ‘\032’ characters as a signal to display the source code for the frame.
-b bps
Set the line speed (baud rate or bits per second) of any serial interface used by _GDBN__ for remote debugging.
-tty=device
Run using device for your program’s standard input and output.
_if__(!_GENERIC__) _include__(gdbinv-s.m4) _fi__(!_GENERIC__)
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quit
To exit _GDBN__, use the quit
command (abbreviated q
), or type
an end-of-file character (usually C-d).
An interrupt (often C-c) will not exit from _GDBN__, but rather will terminate the action of any _GDBN__ command that is in progress and return to _GDBN__ command level. It is safe to type the interrupt character at any time because _GDBN__ does not allow it to take effect until a time when it is safe.
If you’ve been using _GDBN__ to control an attached process or device,
you can release it with the detach
command; see section Debugging an Already-Running Process.
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If you need to execute occasional shell commands during your
debugging session, there’s no need to leave or suspend _GDBN__; you can
just use the shell
command.
shell command string
Directs _GDBN__ to invoke an inferior shell to execute command
string. If it exists, the environment variable SHELL
is used
for the name of the shell to run. Otherwise _GDBN__ uses
/bin/sh
.
The utility make
is often needed in development environments.
You don’t have to use the shell
command for this purpose in _GDBN__:
make make-args
Causes _GDBN__ to execute an inferior make
program with the specified
arguments. This is equivalent to ‘shell make make-args’.
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3.1 Command Syntax | ||
3.2 Getting Help |
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A _GDBN__ command is a single line of input. There is no limit on how long
it can be. It starts with a command name, which is followed by arguments
whose meaning depends on the command name. For example, the command
step
accepts an argument which is the number of times to step,
as in ‘step 5’. You can also use the step
command with
no arguments. Some command names do not allow any arguments.
_GDBN__ command names may always be truncated if that abbreviation is
unambiguous. Other possible command abbreviations are listed in the
documentation for individual commands. In some cases, even ambiguous
abbreviations are allowed; for example, s
is specially defined as
equivalent to step
even though there are other commands whose
names start with s
. You can test abbreviations by using them as
arguments to the help
command.
A blank line as input to _GDBN__ (typing just <RET>) means to
repeat the previous command. Certain commands (for example, run
)
will not repeat this way; these are commands for which unintentional
repetition might cause trouble and which you are unlikely to want to
repeat.
The list
and x
commands, when you repeat them with
<RET>, construct new arguments rather than repeating
exactly as typed. This permits easy scanning of source or memory.
_GDBN__ can also use <RET> in another way: to partition lengthy
output, in a way similar to the common utility more
(see section Screen Size). Since it’s easy to press one <RET> too many
in this situation, _GDBN__ disables command repetition after any command
that generates this sort of display.
A line of input starting with # is a comment; it does nothing. This is useful mainly in command files (See section Command Files).
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You can always ask _GDBN__ itself for information on its commands, using the
command help
.
help
h
You can use help
(abbreviated h
) with no arguments to
display a short list of named classes of commands:
(_GDBP__) help List of classes of commands: running -- Running the program stack -- Examining the stack data -- Examining data breakpoints -- Making program stop at certain points files -- Specifying and examining files status -- Status inquiries support -- Support facilities user-defined -- User-defined commands aliases -- Aliases of other commands obscure -- Obscure features Type "help" followed by a class name for a list of commands in that class. Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (_GDBP__)
help class
Using one of the general help classes as an argument, you can get a
list of the individual commands in that class. For example, here is the
help display for the class status
:
(_GDBP__) help status Status inquiries. List of commands: show -- Generic command for showing things set with "set" info -- Generic command for printing status Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (_GDBP__)
help command
With a command name as help
argument, _GDBN__ will display a
short paragraph on how to use that command.
In addition to help
, you can use the _GDBN__ commands info
and show
to inquire about the state of your program, or the state
of _GDBN__ itself. Each command supports many topics of inquiry; this
manual introduces each of them in the appropriate context. The listings
under info
and under show
in the Index point to
all the sub-commands.
info
This command (abbreviated i
) is for describing the state of your
program; for example, it can list the arguments given to your program
(info args
), the registers currently in use (info
registers
), or the breakpoints you’ve set (info breakpoints
).
You can get a complete list of the info
sub-commands with
help info
.
show
In contrast, show
is for describing the state of _GDBN__ itself.
You can change most of the things you can show
, by using the
related command set
; for example, you can control what number
system is used for displays with set radix
, or simply inquire
which is currently in use with show radix
.
To display all the settable parameters and their current
values, you can use show
with no arguments; you may also use
info set
. Both commands produce the same display.
Here are three miscellaneous show
subcommands, all of which are
exceptional in lacking corresponding set
commands:
show version
Show what version of _GDBN__ is running. You should include this information in _GDBN__ bug-reports. If multiple versions of _GDBN__ are in use at your site, you may occasionally want to make sure what version of _GDBN__ you’re running; as _GDBN__ evolves, new commands are introduced, and old ones may wither away. The version number is also announced when you start _GDBN__ with no arguments.
show copying
Display information about permission for copying _GDBN__.
show warranty
Display the GNU “NO WARRANTY” statement.
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In order to debug a program effectively, you need to generate debugging information when you compile it. This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code.
To request debugging information, specify the ‘-g’ option when you run the compiler.
Many C compilers are unable to handle the ‘-g’ and ‘-O’ options together. Using those compilers, you cannot generate optimized executables containing debugging information.
The GNU C compiler supports ‘-g’ with or without ‘-O’, making it possible to debug optimized code. We recommend that you always use ‘-g’ whenever you compile a program. You may think the program is correct, but there’s no sense in pushing your luck.
Some things do not work as well with ‘-g -O’ as with just ‘-g’, particularly on machines with instruction scheduling. If in doubt, recompile with ‘-g’ alone, and if this fixes the problem, please report it as a bug (including a test case!).
Older versions of the GNU C compiler permitted a variant option ‘-gg’ for debugging information. _GDBN__ no longer supports this format; if your GNU C compiler has this option, do not use it.
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run
r
Use the run
command to start your program under _GDBN__. You
must first specify the program name
_if__(_VXWORKS__)
(except on VxWorks)
_fi__(_VXWORKS__)
with an argument to _GDBN__
(see section Getting In and Out of _GDBN__), or using the file
or exec-file
command (see section Commands to Specify Files).
On targets that support processes, run
creates an inferior
process and makes that process run your program. On other targets,
run
jumps to the start of the program.
The execution of a program is affected by certain information it receives from its superior. _GDBN__ provides ways to specify this information, which you must do before starting the program. (You can change it after starting the program, but such changes will only affect the program the next time you start it.) This information may be divided into four categories:
You specify the arguments to give your program as the arguments of the
run
command. If a shell is available on your target, the shell
is used to pass the arguments, so that you may use normal conventions
(such as wildcard expansion or variable substitution) in
describing the arguments. In Unix systems, you can control which shell
is used with the SHELL
environment variable. See section Your Program’s Arguments.
Your program normally inherits its environment from _GDBN__, but you can
use the _GDBN__ commands set environment
and unset
environment
to change parts of the environment that will be given to
the program. See section Your Program’s Environment.
Your program inherits its working directory from _GDBN__. You can set
_GDBN__’s working directory with the cd
command in _GDBN__.
See section Your Program’s Working Directory.
Your program normally uses the same device for standard input and
standard output as _GDBN__ is using. You can redirect input and output
in the run
command line, or you can use the tty
command to
set a different device for your program.
See section Your Program’s Input and Output.
Warning: While input and output redirection work, you can’t use pipes to pass the output of the program you’re debugging to another program; if you attempt this, _GDBN__ is likely to wind up debugging the wrong program.
When you issue the run
command, your program begins to execute
immediately. See section Stopping and Continuing, for discussion of how to arrange for your
program to stop. Once your program has been started by the run
command (and then stopped), you may evaluate expressions that involve
calls to functions in the inferior, using the print
or
call
commands. See section Examining Data.
If the modification time of your symbol file has changed since the last time _GDBN__ read its symbols, _GDBN__ will discard its symbol table and re-read it. In this process, it tries to retain your current breakpoints.
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The arguments to your program can be specified by the arguments of the
run
command. They are passed to a shell, which expands wildcard
characters and performs redirection of I/O, and thence to the program.
_GDBN__ uses the shell indicated by your environment variable
SHELL
if it exists; otherwise, _GDBN__ uses /bin/sh
.
run
with no arguments uses the same arguments used by the previous
run
, or those set by the set args
command.
set args
Specify the arguments to be used the next time your program is run. If
set args
has no arguments, run
will execute your program
with no arguments. Once you have run your program with arguments,
using set args
before the next run
is the only way to run
it again without arguments.
show args
Show the arguments to give your program when it is started.
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The environment consists of a set of environment variables and their values. Environment variables conventionally record such things as your user name, your home directory, your terminal type, and your search path for programs to run. Usually you set up environment variables with the shell and they are inherited by all the other programs you run. When debugging, it can be useful to try running the program with a modified environment without having to start _GDBN__ over again.
path directory
Add directory to the front of the PATH
environment variable
(the search path for executables), for both _GDBN__ and your program.
You may specify several directory names, separated by ‘:’ or
whitespace. If directory is already in the path, it is moved to
the front, so it will be searched sooner.
You can use the string ‘$cwd’ to refer to whatever is the current
working directory at the time _GDBN__ searches the path. If you use
‘.’ instead, it refers to the directory where you executed the
path
command. _GDBN__ fills in the current path where needed in
the directory argument, before adding it to the search path.
show paths
Display the list of search paths for executables (the PATH
environment variable).
show environment [varname]
Print the value of environment variable varname to be given to
your program when it starts. If you don’t supply varname,
print the names and values of all environment variables to be given to
your program. You can abbreviate environment
as env
.
set environment varname [=] value
Sets environment variable varname to value. The value changes for your program only, not for _GDBN__ itself. value may be any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself. The value parameter is optional; if it is eliminated, the variable is set to a null value.
For example, this command:
set env USER = foo
tells a Unix program, when subsequently run, that its user is named ‘foo’. (The spaces around ‘=’ are used for clarity here; they are not actually required.)
unset environment varname
Remove variable varname from the environment to be passed to your
program. This is different from ‘set env varname =’;
unset environment
removes the variable from the environment,
rather than assigning it an empty value.
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Each time you start your program with run
, it inherits its
working directory from the current working directory of _GDBN__. _GDBN__’s
working directory is initially whatever it inherited from its parent
process (typically the shell), but you can specify a new working
directory in _GDBN__ with the cd
command.
The _GDBN__ working directory also serves as a default for the commands that specify files for _GDBN__ to operate on. See section Commands to Specify Files.
cd directory
Set _GDBN__’s working directory to directory.
pwd
Print _GDBN__’s working directory.
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By default, the program you run under _GDBN__ does input and output to the same terminal that _GDBN__ uses. _GDBN__ switches the terminal to its own terminal modes to interact with you, but it records the terminal modes your program was using and switches back to them when you continue running your program.
info terminal
Displays _GDBN__’s recorded information about the terminal modes your program is using.
You can redirect the program’s input and/or output using shell
redirection with the run
command. For example,
_0__
run > outfile _1__
starts the program, diverting its output to the file ‘outfile’.
Another way to specify where the program should do input and output is
with the tty
command. This command accepts a file name as
argument, and causes this file to be the default for future run
commands. It also resets the controlling terminal for the child
process, for future run
commands. For example,
tty /dev/ttyb
directs that processes started with subsequent run
commands
default to do input and output on the terminal ‘/dev/ttyb’ and have
that as their controlling terminal.
An explicit redirection in run
overrides the tty
command’s
effect on the input/output device, but not its effect on the controlling
terminal.
When you use the tty
command or redirect input in the run
command, only the input for your program is affected. The input
for _GDBN__ still comes from your terminal.
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attach process-id
This command
attaches to a running process—one that was started outside _GDBN__.
(info files
will show your active targets.) The command takes as
argument a process ID. The usual way to find out the process-id of
a Unix process is with the ps
utility, or with the ‘jobs -l’
shell command.
attach
will not repeat if you press <RET> a second time after
executing the command.
To use attach
, you must be debugging in an environment which
supports processes. You must also have permission to send the process a
signal, and it must have the same effective user ID as the _GDBN__
process.
When using attach
, you should first use the file
command
to specify the program running in the process and load its symbol table.
See section Commands to Specify Files.
The first thing _GDBN__ does after arranging to debug the specified
process is to stop it. You can examine and modify an attached process
with all the _GDBN__ commands that are ordinarily available when you start
processes with run
. You can insert breakpoints; you can step and
continue; you can modify storage. If you would rather the process
continue running, you may use the continue
command after
attaching _GDBN__ to the process.
detach
When you have finished debugging the attached process, you can use the
detach
command to release it from _GDBN__’s control. Detaching
the process continues its execution. After the detach
command,
that process and _GDBN__ become completely independent once more, and you
are ready to attach
another process or start one with run
.
detach
will not repeat if you press <RET> again after
executing the command.
If you exit _GDBN__ or use the run
command while you have an attached
process, you kill that process. By default, you will be asked for
confirmation if you try to do either of these things; you can control
whether or not you need to confirm by using the set confirm
command
(see section Optional Warnings and Messages).
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kill
Kill the child process in which your program is running under _GDBN__.
This command is useful if you wish to debug a core dump instead of a running process. _GDBN__ ignores any core dump file while your program is running.
On some operating systems, a program can’t be executed outside _GDBN__
while you have breakpoints set on it inside _GDBN__. You can use the
kill
command in this situation to permit running the program
outside the debugger.
The kill
command is also useful if you wish to recompile and
relink the program, since on many systems it is impossible to modify an
executable file while it is running in a process. In this case, when you
next type run
, _GDBN__ will notice that the file has changed, and
will re-read the symbol table (while trying to preserve your current
breakpoint settings).
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The principal purpose of using a debugger is so that you can stop your program before it terminates; or so that, if the program runs into trouble, you can investigate and find out why.
Inside _GDBN__, your program may stop for any of several reasons, such
as a signal, a breakpoint, or reaching a new line after a _GDBN__
command such as step
. You may then examine and change
variables, set new breakpoints or remove old ones, and then continue
execution. Usually, the messages shown by _GDBN__ provide ample
explanation of the status of your program—but you can also explicitly
request this information at any time.
info program
Display information about the status of your program: whether it is running or not, what process it is, and why it stopped.
5.1 Breakpoints, Watchpoints, and Exceptions | ||
5.2 Continuing and Stepping | Resuming Execution | |
5.3 Signals |
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A breakpoint makes your program stop whenever a certain point in
the program is reached. For each breakpoint, you can add various
conditions to control in finer detail whether the program will stop.
You can set breakpoints with the break
command and its variants
(see section Setting Breakpoints), to specify the place where the program should stop
by line number, function name or exact address in the program. In
languages with exception handling (such as GNU C++), you can also set
breakpoints where an exception is raised (see section Breakpoints and Exceptions).
A watchpoint is a special breakpoint that stops your program when the value of an expression changes. You must use a different command to set watchpoints (see section Setting Watchpoints), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.
Each breakpoint or watchpoint is assigned a number when it is created; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be enabled or disabled; if disabled, it has no effect on the program until you enable it again.
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Breakpoints are set with the break
command (abbreviated b
).
You have several ways to say where the breakpoint should go.
break function
Set a breakpoint at entry to function function. When using source languages that permit overloading of symbols, such as C++, function may refer to more than one possible place to break. See section Breakpoint Menus, for a discussion of that situation.
break +offset
break -offset
Set a breakpoint some number of lines forward or back from the position at which execution stopped in the currently selected frame.
break linenum
Set a breakpoint at line linenum in the current source file. That file is the last file whose source text was printed. This breakpoint will stop the program just before it executes any of the code on that line.
break filename:linenum
Set a breakpoint at line linenum in source file filename.
break filename:function
Set a breakpoint at entry to function function found in file filename. Specifying a file name as well as a function name is superfluous except when multiple files contain similarly named functions.
break *address
Set a breakpoint at address address. You can use this to set breakpoints in parts of the program which do not have debugging information or source files.
break
When called without any arguments, break
sets a breakpoint at the
next instruction to be executed in the selected stack frame
(see section Examining the Stack). In any selected frame but the innermost, this will
cause the program to stop as soon as control returns to that frame.
This is similar to the effect of a finish
command in the frame
inside the selected frame—except that finish
doesn’t leave an
active breakpoint. If you use break
without an argument in the
innermost frame, _GDBN__ will stop the next time it reaches the current
location; this may be useful inside loops.
_GDBN__ normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies whether or not the breakpoint already existed when the program stopped.
break … if cond
Set a breakpoint with condition cond; evaluate the expression cond each time the breakpoint is reached, and stop only if the value is nonzero—that is, if cond evaluates as true. ‘…’ stands for one of the possible arguments described above (or no argument) specifying where to break. See section Break Conditions, for more information on breakpoint conditions.
tbreak args
Set a breakpoint enabled only for one stop. args are the
same as for the break
command, and the breakpoint is set in the same
way, but the breakpoint is automatically disabled the first time it
is hit. See section Disabling Breakpoints.
rbreak regex
Set breakpoints on all functions matching the regular expression
regex. This command
sets an unconditional breakpoint on all matches, printing a list of all
breakpoints it set. Once these breakpoints are set, they are treated
just like the breakpoints set with the break
command. They can
be deleted, disabled, made conditional, etc., in the standard ways.
When debugging C++ programs, rbreak
is useful for setting
breakpoints on overloaded functions that are not members of any special
classes.
info breakpoints [n]
info break [n]
Print a list of all breakpoints (but not watchpoints) set and not
deleted, showing their numbers, where in the program they are, and any
special features in use for them. Disabled breakpoints are included in
the list, but marked as disabled. info break
with a breakpoint
number n as argument lists only that breakpoint. The convenience
variable $_
and the default examining-address for the x
command are set to the address of the last breakpoint listed
(see section Examining Memory). The equivalent command for watchpoints is info
watch
.
_GDBN__ allows you to set any number of breakpoints at the same place in the program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (see section Break Conditions).
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You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen.
Watchpoints currently execute two orders of magnitude more slowly than other breakpoints, but this can well be worth it to catch errors where you have no clue what part of your program is the culprit. Some processors provide special hardware to support watchpoint evaluation; future releases of _GDBN__ will use such hardware if it is available.
watch expr
Set a watchpoint for an expression.
info watchpoints
This command prints a list of watchpoints; it is otherwise similar to
info break
.
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Some languages, such as GNU C++, implement exception handling. You can use _GDBN__ to examine what caused the program to raise an exception, and to list the exceptions the program is prepared to handle at a given point in time.
catch exceptions
You can set breakpoints at active exception handlers by using the
catch
command. exceptions is a list of names of exceptions
to catch.
You can use info catch
to list active exception handlers;
see section Information About a Frame.
There are currently some limitations to exception handling in _GDBN__. These will be corrected in a future release.
Sometimes catch
is not the best way to debug exception handling:
if you need to know exactly where an exception is raised, it’s better to
stop before the exception handler is called, since that way you
can see the stack before any unwinding takes place. If you set a
breakpoint in an exception handler instead, it may not be easy to find
out where the exception was raised.
To stop just before an exception handler is called, you need some
knowledge of the implementation. In the case of GNU C++, exceptions are
raised by calling a library function named __raise_exception
which has the following ANSI C interface:
/* addr is where the exception identifier is stored. ID is the exception identifier. */ void __raise_exception (void **addr, void *id);
To make the debugger catch all exceptions before any stack
unwinding takes place, set a breakpoint on __raise_exception
(see section Breakpoints, Watchpoints, and Exceptions).
With a conditional breakpoint (See section Break Conditions) that depends on the value of id, you can stop your program when a specific exception is raised. You can use multiple conditional breakpoints to stop the program when any of a number of exceptions are raised.
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It is often necessary to eliminate a breakpoint or watchpoint once it has done its job and you no longer want the program to stop there. This is called deleting the breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten.
With the clear
command you can delete breakpoints according to
where they are in the program. With the delete
command you can
delete individual breakpoints or watchpoints by specifying their
breakpoint numbers.
It is not necessary to delete a breakpoint to proceed past it. _GDBN__ automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.
clear
Delete any breakpoints at the next instruction to be executed in the selected stack frame (see section Selecting a Frame). When the innermost frame is selected, this is a good way to delete a breakpoint that the program just stopped at.
clear function
clear filename:function
Delete any breakpoints set at entry to the function function.
clear linenum
clear filename:linenum
Delete any breakpoints set at or within the code of the specified line.
delete [breakpoints] [bnums…]
Delete the breakpoints or watchpoints of the numbers specified as
arguments. If no argument is specified, delete all breakpoints (_GDBN__
asks confirmation, unless you’ve set confirm off
). You
can abbreviate this command as d
.
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Rather than deleting a breakpoint or watchpoint, you might prefer to disable it. This makes the breakpoint inoperative as if it had been deleted, but remembers the information on the breakpoint so that you can enable it again later.
You disable and enable breakpoints and watchpoints with the
enable
and disable
commands, optionally specifying one or
more breakpoint numbers as arguments. Use info break
or
info watch
to print a list of breakpoints or watchpoints if you
don’t know which numbers to use.
A breakpoint or watchpoint can have any of four different states of enablement:
break
command starts out in this state.
tbreak
command starts out in this state.
You can use the following commands to enable or disable breakpoints and watchpoints:
disable [breakpoints] [bnums…]
Disable the specified breakpoints—or all breakpoints, if none are
listed. A disabled breakpoint has no effect but is not forgotten. All
options such as ignore-counts, conditions and commands are remembered in
case the breakpoint is enabled again later. You may abbreviate
disable
as dis
.
enable [breakpoints] [bnums…]
Enable the specified breakpoints (or all defined breakpoints). They become effective once again in stopping the program.
enable [breakpoints] once bnums…
Enable the specified breakpoints temporarily. Each will be disabled again the next time it stops the program.
enable [breakpoints] delete bnums…
Enable the specified breakpoints to work once and then die. Each of the breakpoints will be deleted the next time it stops the program.
Save for a breakpoint set with tbreak
(see section Setting Breakpoints),
breakpoints that you set are initially enabled; subsequently, they become
disabled or enabled only when you use one of the commands above. (The
command until
can set and delete a breakpoint of its own, but it
will not change the state of your other breakpoints;
see section Continuing and Stepping.)
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The simplest sort of breakpoint breaks every time the program reaches a specified place. You can also specify a condition for a breakpoint. A condition is just a Boolean expression in your programming language. (See section Expressions). A breakpoint with a condition evaluates the expression each time the program reaches it, and the program stops only if the condition is true.
This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated—that is, when the condition is false. In C, if you want to test an assertion expressed by the condition assert, you should set the condition ‘! assert’ on the appropriate breakpoint.
Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow—but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one.
Break conditions ca have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, _GDBN__ might see the other breakpoint first and stop the program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible for the purpose of performing side effects when a breakpoint is reached (see section Breakpoint Command Lists).
Break conditions can be specified when a breakpoint is set, by using
‘if’ in the arguments to the break
command. See section Setting Breakpoints.
They can also be changed at any time with the condition
command.
The watch
command doesn’t recognize the if
keyword;
condition
is the only way to impose a further condition on a
watchpoint.
condition bnum expression
Specify expression as the break condition for breakpoint or
watchpoint number bnum. From now on, this breakpoint will stop
the program only if the value of expression is true (nonzero, in
C). When you use condition
, _GDBN__ checks expression
immediately for syntactic correctness, and to determine whether symbols
in it have referents in the context of your breakpoint. _GDBN__ does
not actually evaluate expression at the time the condition
command is given, however. See section Expressions.
condition bnum
Remove the condition from breakpoint number bnum. It becomes an ordinary unconditional breakpoint.
A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times. This is so useful that there is a special way to do it, using the ignore count of the breakpoint. Every breakpoint has an ignore count, which is an integer. Most of the time, the ignore count is zero, and therefore has no effect. But if the program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues. As a result, if the ignore count value is n, the breakpoint will not stop the next n times it is reached.
ignore bnum count
Set the ignore count of breakpoint number bnum to count. The next count times the breakpoint is reached, your program’s execution will not stop; other than to decrement the ignore count, _GDBN__ takes no action.
To make the breakpoint stop the next time it is reached, specify a count of zero.
continue count
c count
fg count
Continue execution of the program, setting the ignore count of the breakpoint that the program stopped at to count minus one. Thus, the program will not stop at this breakpoint until the count’th time it is reached.
An argument to this command is meaningful only when the program stopped
due to a breakpoint. At other times, the argument to continue
is
ignored.
The synonym fg
is provided purely for convenience, and has
exactly the same behavior as other forms of the command.
If a breakpoint has a positive ignore count and a condition, the condition is not checked. Once the ignore count reaches zero, the condition will be checked.
You could achieve the effect of the ignore count with a condition such as _0__‘$foo-- <= 0’_1__ using a debugger convenience variable that is decremented each time. See section Convenience Variables.
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You can give any breakpoint (or watchpoint) a series of commands to execute when the program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints.
commands [bnum]
… command-list …
end
Specify a list of commands for breakpoint number bnum. The commands
themselves appear on the following lines. Type a line containing just
end
to terminate the commands.
To remove all commands from a breakpoint, type commands
followed
immediately by end
; that is, give no commands.
With no bnum argument, commands
refers to the last
breakpoint or watchpoint set (not to the breakpoint most recently
encountered).
Pressing <RET> as a means of repeating the last _GDBN__ command is disabled within a command-list.
You can use breakpoint commands to start the program up again. Simply
use the continue
command, or step
, or any other command
that resumes execution. Subsequent commands in the command list are
ignored.
If the first command specified is silent
, the usual message about
stopping at a breakpoint is not printed. This may be desirable for
breakpoints that are to print a specific message and then continue.
If the remaining commands too print nothing, you will see no sign that
the breakpoint was reached at all. silent
is meaningful only
at the beginning of a breakpoint command list.
The commands echo
and output
that allow you to print precisely
controlled output are often useful in silent breakpoints. See section Commands for Controlled Output.
For example, here is how you could use breakpoint commands to print the
value of x
at entry to foo
whenever x
is positive.
_0__
break foo if x>0 commands silent echo x is\040 output x echo \n cont end _1__
One application for breakpoint commands is to compensate for one bug so
you can test for another. Put a breakpoint just after the erroneous line
of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them. End with the continue
command
so that the program does not stop, and start with the silent
command so that no output is produced. Here is an example:
break 403 commands silent set x = y + 4 cont end
One deficiency in the operation of automatically continuing breakpoints under Unix appears when your program uses raw mode for the terminal. _GDBN__ switches back to its own terminal modes (not raw) before executing commands, and then must switch back to raw mode when your program is continued. This causes any pending terminal input to be lost.
Under Unix, you can get around this problem by writing actions into the breakpoint condition rather than in commands. For example
condition 5 (x = y + 4), 0
specifies a condition expression (See section Expressions) that will change
x
as needed, then always have the value zero so the program will
not stop. No input is lost here, because _GDBN__ evaluates break
conditions without changing the terminal modes. When you want to have
nontrivial conditions for performing the side effects, the operators
‘&&’, ‘||’ and ‘?…:’ may be useful.
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Some programming languages (notably C++) permit a single function name
to be defined several times, for application in different contexts.
This is called overloading. When a function name is overloaded,
‘break function’ is not enough to tell _GDBN__ where you
want a breakpoint. _GDBN__ offers you a menu of numbered choices for
different possible breakpoints, and waits for your selection with the
prompt ‘>’. The first two options are always ‘[0] cancel’
and ‘[1] all’. Typing 1 sets a breakpoint at each
definition of function, and typing 0 aborts the
break
command without setting any new breakpoints.
For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol String::after
.
We choose three particular definitions of that function name:
(_GDBP__) b String::after [0] cancel [1] all [2] file:String.cc; line number:867 [3] file:String.cc; line number:860 [4] file:String.cc; line number:875 [5] file:String.cc; line number:853 [6] file:String.cc; line number:846 [7] file:String.cc; line number:735 > 2 4 6 Breakpoint 1 at 0xb26c: file String.cc, line 867. Breakpoint 2 at 0xb344: file String.cc, line 875. Breakpoint 3 at 0xafcc: file String.cc, line 846. Multiple breakpoints were set. Use the "delete" command to delete unwanted breakpoints. (_GDBP__)
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Under some operating systems, breakpoints cannot be used in a program if any other process is running that program. In this situation, attempting to run or continue a program with a breakpoint causes _GDBN__ to stop the other process.
When this happens, you have three ways to proceed:
exec-file
command to specify that _GDBN__
should run the program under that name. Then start the program again.
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Continuing means resuming program execution until your program
completes normally. In contrast, stepping means executing just
one more “step” of your program, where “step” may mean either one
line of source code, or one machine instruction (depending on what
particular command you use). Either when continuing
or when stepping, the program may stop even sooner, due to a breakpoint
or to a signal. (If due to a signal, you may want to use handle
,
or use ‘signal 0’ to resume execution; see section Signals.)
continue [ignore-count]
Resume program execution, at the address where the program last stopped;
any breakpoints set at that address are bypassed. The optional argument
ignore-count allows you to specify a further number of times to
ignore a breakpoint at this location; its effect is like that of
ignore
(see section Break Conditions).
To resume execution at a different place, you can use return
(see section Returning from a Function) to go back to the calling function; or jump
(see section Continuing at a Different Address) to go to an arbitrary location in your program.
A typical technique for using stepping is to set a breakpoint (see section Breakpoints, Watchpoints, and Exceptions) at the beginning of the function or the section of the program in which a problem is believed to lie, run the program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.
step
Continue running the program until control reaches a different source
line, then stop it and return control to _GDBN__. This command is
abbreviated s
.
Warning: If you use the
step
command while control is within a function that was compiled without debugging information, execution will proceed until control reaches another function.
step count
Continue running as in step
, but do so count times. If a
breakpoint is reached or a signal not related to stepping occurs before
count steps, stepping stops right away.
next [count]
Continue to the next source line in the current (innermost) stack frame.
Similar to step
, but any function calls appearing within the line
of code are executed without stopping. Execution stops when control
reaches a different line of code at the stack level which was executing
when the next
command was given. This command is abbreviated
n
.
An argument count is a repeat count, as for step
.
next
within a function that lacks debugging information acts like
step
, but any function calls appearing within the code of the
function are executed without stopping.
finish
Continue running until just after function in the selected stack frame returns. Print the returned value (if any).
Contrast this with the return
command (see section Returning from a Function).
until
u
Continue running until a source line past the current line, in the
current stack frame, is reached. This command is used to avoid single
stepping through a loop more than once. It is like the next
command, except that when until
encounters a jump, it
automatically continues execution until the program counter is greater
than the address of the jump.
This means that when you reach the end of a loop after single stepping
though it, until
will cause the program to continue execution
until the loop is exited. In contrast, a next
command at the end
of a loop will simply step back to the beginning of the loop, which
would force you to step through the next iteration.
until
always stops the program if it attempts to exit the current
stack frame.
until
may produce somewhat counterintuitive results if the order
of machine code does not match the order of the source lines. For
example, in the following excerpt from a debugging session, the f
(frame
) command shows that execution is stopped at line
206
; yet when we use until
, we get to line 195
:
(_GDBP__) f #0 main (argc=4, argv=0xf7fffae8) at m4.c:206 206 expand_input(); (_GDBP__) until 195 for ( ; argc > 0; NEXTARG) {
This happened because, for execution efficiency, the compiler had
generated code for the loop closure test at the end, rather than the
start, of the loop—even though the test in a C for
-loop is
written before the body of the loop. The until
command appeared
to step back to the beginning of the loop when it advanced to this
expression; however, it has not really gone to an earlier
statement—not in terms of the actual machine code.
until
with no argument works by means of single
instruction stepping, and hence is slower than until
with an
argument.
until location
u location
Continue running the program until either the specified location is
reached, or the current stack frame returns. location
is any of the forms of argument acceptable to break
(see section Setting Breakpoints). This form of the command uses breakpoints, and
hence is quicker than until
without an argument.
stepi
si
Execute one machine instruction, then stop and return to the debugger.
It is often useful to do ‘display/i $pc’ when stepping by machine instructions. This will cause the next instruction to be executed to be displayed automatically at each stop. See section Automatic Display.
An argument is a repeat count, as in step
.
nexti
ni
Execute one machine instruction, but if it is a function call, proceed until the function returns.
An argument is a repeat count, as in next
.
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A signal is an asynchronous event that can happen in a program. The
operating system defines the possible kinds of signals, and gives each
kind a name and a number. For example, in Unix SIGINT
is the
signal a program gets when you type an interrupt (often C-c);
SIGSEGV
is the signal a program gets from referencing a place in
memory far away from all the areas in use; SIGALRM
occurs when
the alarm clock timer goes off (which happens only if the program has
requested an alarm).
Some signals, including SIGALRM
, are a normal part of the
functioning of the program. Others, such as SIGSEGV
, indicate
errors; these signals are fatal (kill the program immediately) if the
program has not specified in advance some other way to handle the signal.
SIGINT
does not indicate an error in the program, but it is normally
fatal so it can carry out the purpose of the interrupt: to kill the program.
_GDBN__ has the ability to detect any occurrence of a signal in the program running under _GDBN__’s control. You can tell _GDBN__ in advance what to do for each kind of signal.
Normally, _GDBN__ is set up to ignore non-erroneous signals like SIGALRM
(so as not to interfere with their role in the functioning of the program)
but to stop the program immediately whenever an error signal happens.
You can change these settings with the handle
command.
info signals
Print a table of all the kinds of signals and how _GDBN__ has been told to handle each one. You can use this to see the signal numbers of all the defined types of signals.
handle signal keywords…
Change the way _GDBN__ handles signal signal. signal can be the number of a signal or its name (with or without the ‘SIG’ at the beginning). The keywords say what change to make.
The keywords allowed by the handle
command can be abbreviated.
Their full names are:
nostop
_GDBN__ should not stop the program when this signal happens. It may still print a message telling you that the signal has come in.
stop
_GDBN__ should stop the program when this signal happens. This implies
the print
keyword as well.
print
_GDBN__ should print a message when this signal happens.
noprint
_GDBN__ should not mention the occurrence of the signal at all. This
implies the nostop
keyword as well.
pass
_GDBN__ should allow the program to see this signal; the program will be able to handle the signal, or may be terminated if the signal is fatal and not handled.
nopass
_GDBN__ should not allow the program to see this signal.
When a signal has been set to stop the program, the program cannot see the
signal until you continue. It will see the signal then, if pass
is
in effect for the signal in question at that time. In other words,
after _GDBN__ reports a signal, you can use the handle
command with
pass
or nopass
to control whether that signal will be seen by
the program when you later continue it.
You can also use the signal
command to prevent the program from
seeing a signal, or cause it to see a signal it normally would not see,
or to give it any signal at any time. For example, if the program stopped
due to some sort of memory reference error, you might store correct
values into the erroneous variables and continue, hoping to see more
execution; but the program would probably terminate immediately as
a result of the fatal signal once it sees the signal. To prevent this,
you can continue with ‘signal 0’. See section Giving the Program a Signal.
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When your program has stopped, the first thing you need to know is where it stopped and how it got there.
Each time your program performs a function call, the information about where in the program the call was made from is saved in a block of data called a stack frame. The frame also contains the arguments of the call and the local variables of the function that was called. All the stack frames are allocated in a region of memory called the call stack.
When your program stops, the _GDBN__ commands for examining the stack allow you to see all of this information.
One of the stack frames is selected by _GDBN__ and many _GDBN__ commands refer implicitly to the selected frame. In particular, whenever you ask _GDBN__ for the value of a variable in the program, the value is found in the selected frame. There are special _GDBN__ commands to select whichever frame you are interested in.
When the program stops, _GDBN__ automatically selects the currently executing
frame and describes it briefly as the frame
command does
(see section Information About a Frame).
6.1 Stack Frames | ||
6.2 Backtraces | ||
6.3 Selecting a Frame | ||
6.4 Information About a Frame | Information on a Frame |
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The call stack is divided up into contiguous pieces called stack frames, or frames for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function’s local variables, and the address at which the function is executing.
When your program is started, the stack has only one frame, that of the
function main
. This is called the initial frame or the
outermost frame. Each time a function is called, a new frame is
made. Each time a function returns, the frame for that function invocation
is eliminated. If a function is recursive, there can be many frames for
the same function. The frame for the function in which execution is
actually occurring is called the innermost frame. This is the most
recently created of all the stack frames that still exist.
Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one of those bytes whose address serves as the address of the frame. Usually this address is kept in a register called the frame pointer register while execution is going on in that frame.
_GDBN__ assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by _GDBN__ to give you a way of designating stack frames in _GDBN__ commands.
Some compilers allow functions to be compiled so that they operate
without stack frames. (For example, the _GCC__
option
‘-fomit-frame-pointer’ will generate functions without a frame.)
This is occasionally done with heavily used library functions to save
the frame setup time. _GDBN__ has limited facilities for dealing with
these function invocations. If the innermost function invocation has no
stack frame, _GDBN__ will nevertheless regard it as though it had a
separate frame, which is numbered zero as usual, allowing correct
tracing of the function call chain. However, _GDBN__ has no provision
for frameless functions elsewhere in the stack.
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A backtrace is a summary of how the program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.
backtrace
bt
Print a backtrace of the entire stack: one line per frame for all frames in the stack.
You can stop the backtrace at any time by typing the system interrupt character, normally C-c.
backtrace n
bt n
Similar, but print only the innermost n frames.
backtrace -n
bt -n
Similar, but print only the outermost n frames.
The names where
and info stack
(abbreviated info s
)
are additional aliases for backtrace
.
Each line in the backtrace shows the frame number and the function name.
The program counter value is also shown—unless you use set
print address off
. The backtrace also shows the source file name and
line number, as well as the arguments to the function. The program
counter value is omitted if it is at the beginning of the code for that
line number.
Here is an example of a backtrace. It was made with the command ‘bt 3’, so it shows the innermost three frames.
#0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) at builtin.c:993 #1 0x6e38 in expand_macro (sym=0x2b600) at macro.c:242 #2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08) at macro.c:71 (More stack frames follow...)
The display for frame zero doesn’t begin with a program counter
value, indicating that the program has stopped at the beginning of the
code for line 993
of builtin.c
.
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Most commands for examining the stack and other data in the program work on whichever stack frame is selected at the moment. Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected.
frame n
f n
Select frame number n. Recall that frame zero is the innermost
(currently executing) frame, frame one is the frame that called the
innermost one, and so on. The highest-numbered frame is main
’s
frame.
frame addr
f addr
Select the frame at address addr. This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for _GDBN__ to assign numbers properly to all frames. In addition, this can be useful when the program has multiple stacks and switches between them.
_if__(_SPARC__)
On the SPARC architecture, frame
needs two addresses to
select an arbitrary frame: a frame pointer and a stack pointer.
_fi__(_SPARC__)
up n
Move n frames up the stack. For positive numbers n, this advances toward the outermost frame, to higher frame numbers, to frames that have existed longer. n defaults to one.
down n
Move n frames down the stack. For positive numbers n, this
advances toward the innermost frame, to lower frame numbers, to frames
that were created more recently. n defaults to one. You may
abbreviate down
as do
.
All of these commands end by printing two lines of output describing the frame. The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame. The second line shows the text of that source line. For example:
(_GDBP__) up #1 0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc) at env.c:10 10 read_input_file (argv[i]);
After such a printout, the list
command with no arguments will print
ten lines centered on the point of execution in the frame. See section Printing Source Lines.
up-silently n
down-silently n
These two commands are variants of up
and down
,
respectively; they differ in that they do their work silently, without
causing display of the new frame. They are intended primarily for use
in _GDBN__ command scripts, where the output might be unnecessary and
distracting.
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There are several other commands to print information about the selected stack frame.
frame
f
When used without any argument, this command does not change which frame
is selected, but prints a brief description of the currently
selected stack frame. It can be abbreviated f
. With an
argument, this command is used to select a stack frame (see section Selecting a Frame).
info frame
info f
This command prints a verbose description of the selected stack frame, including the address of the frame, the addresses of the next frame down (called by this frame) and the next frame up (caller of this frame), the language that the source code corresponding to this frame was written in, the address of the frame’s arguments, the program counter saved in it (the address of execution in the caller frame), and which registers were saved in the frame. The verbose description is useful when something has gone wrong that has made the stack format fail to fit the usual conventions.
info frame addr
info f addr
Print a verbose description of the frame at address addr, without selecting that frame. The selected frame remains unchanged by this command.
info args
Print the arguments of the selected frame, each on a separate line.
info locals
Print the local variables of the selected frame, each on a separate line. These are all variables declared static or automatic within all program blocks that execution in this frame is currently inside of.
info catch
Print a list of all the exception handlers that are active in the
current stack frame at the current point of execution. To see other
exception handlers, visit the associated frame (using the up
,
down
, or frame
commands); then type info catch
.
See section Breakpoints and Exceptions.
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_GDBN__ can print parts of your program’s source, since the debugging information recorded in your program tells _GDBN__ what source files were used to built it. When your program stops, _GDBN__ spontaneously prints the line where it stopped. Likewise, when you select a stack frame (see section Selecting a Frame), _GDBN__ prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command.
If you use _GDBN__ through its GNU Emacs interface, you may prefer to use Emacs facilities to view source; see section Using _GDBN__ under GNU Emacs.
7.1 Printing Source Lines | ||
7.2 Searching Source Files | ||
7.3 Specifying Source Directories | ||
7.4 Source and Machine Code |
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To print lines from a source file, use the list
command
(abbreviated l
). There are several ways to specify what part
of the file you want to print.
Here are the forms of the list
command most commonly used:
list linenum
Print lines centered around line number linenum in the current source file.
list function
Print lines centered around the beginning of function function.
list
Print more lines. If the last lines printed were printed with a
list
command, this prints lines following the last lines
printed; however, if the last line printed was a solitary line printed
as part of displaying a stack frame (see section Examining the Stack), this prints
lines centered around that line.
list -
Print lines just before the lines last printed.
By default, _GDBN__ prints ten source lines with any of these forms of
the list
command. You can change this using set listsize
:
set listsize count
Make the list
command display count source lines (unless
the list
argument explicitly specifies some other number).
show listsize
Display the number of lines that list
will currently display by
default.
Repeating a list
command with <RET> discards the argument,
so it is equivalent to typing just list
. This is more useful
than listing the same lines again. An exception is made for an
argument of ‘-’; that argument is preserved in repetition so that
each repetition moves up in the source file.
In general, the list
command expects you to supply zero, one or two
linespecs. Linespecs specify source lines; there are several ways
of writing them but the effect is always to specify some source line.
Here is a complete description of the possible arguments for list
:
list linespec
Print lines centered around the line specified by linespec.
list first,last
Print lines from first to last. Both arguments are linespecs.
list ,last
Print lines ending with last.
list first,
Print lines starting with first.
list +
Print lines just after the lines last printed.
list -
Print lines just before the lines last printed.
list
As described in the preceding table.
Here are the ways of specifying a single source line—all the kinds of linespec.
number
Specifies line number of the current source file.
When a list
command has two linespecs, this refers to
the same source file as the first linespec.
+offset
Specifies the line offset lines after the last line printed.
When used as the second linespec in a list
command that has
two, this specifies the line offset lines down from the
first linespec.
-offset
Specifies the line offset lines before the last line printed.
filename:number
Specifies line number in the source file filename.
function
Specifies the line of the open-brace that begins the body of the function function.
filename:function
Specifies the line of the open-brace that begins the body of the function function in the file filename. You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files.
*address
Specifies the line containing the program address address. address may be any expression.
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There are two commands for searching through the current source file for a regular expression.
forward-search regexp
search regexp
The command ‘forward-search regexp’ checks each line, starting
with the one following the last line listed, for a match for regexp.
It lists the line that is found. You can abbreviate the command name
as fo
. The synonym ‘search regexp’ is also supported.
reverse-search regexp
The command ‘reverse-search regexp’ checks each line, starting
with the one before the last line listed and going backward, for a match
for regexp. It lists the line that is found. You can abbreviate
this command as rev
.
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Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names. Even when they do, the directories could be moved between the compilation and your debugging session. _GDBN__ has a list of directories to search for source files; this is called the source path. Each time _GDBN__ wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name. Note that the executable search path is not used for this purpose. Neither is the current working directory, unless it happens to be in the source path.
If _GDBN__ can’t find a source file in the source path, and the object program records a directory, _GDBN__ tries that directory too. If the source path is empty, and there is no record of the compilation directory, _GDBN__ will, as a last resort, look in the current directory.
Whenever you reset or rearrange the source path, _GDBN__ will clear out any information it has cached about where source files are found, where each line is in the file, etc.
When you start _GDBN__, its source path is empty.
To add other directories, use the directory
command.
directory dirname …
Add directory dirname to the front of the source path. Several directory names may be given to this command, separated by ‘:’ or whitespace. You may specify a directory that is already in the source path; this moves it forward, so it will be searched sooner.
You can use the string ‘$cdir’ to refer to the compilation directory (if one is recorded), and ‘$cwd’ to refer to the current working directory. ‘$cwd’ is not the same as ‘.’—the former tracks the current working directory as it changes during your _GDBN__ session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path.
directory
Reset the source path to empty again. This requires confirmation.
show directories
Print the source path: show which directories it contains.
If your source path is cluttered with directories that are no longer of interest, _GDBN__ may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows:
directory
with no argument to reset the source path to empty.
directory
with suitable arguments to reinstall the
directories you want in the source path. You can add all the
directories in one command.
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You can use the command info line
to map source lines to program
addresses (and viceversa), and the command disassemble
to display
a range of addresses as machine instructions.
info line linespec
Print the starting and ending addresses of the compiled code for
source line linespec. You can specify source lines in any of the
ways understood by the list
command (see section Printing Source Lines).
For example, we can use info line
to inquire on where the object
code for the first line of function m4_changequote
lies:
(_GDBP__) info line m4_changecom Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.
We can also inquire (using *addr
as the form for
linespec) what source line covers a particular address:
(_GDBP__) info line *0x63ff Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.
After info line
, the default address for the x
command is changed to the starting address of the line, so that
‘x/i’ is sufficient to begin examining the machine code
(see section Examining Memory). Also, this address is saved as the value of the
convenience variable $_
(see section Convenience Variables).
disassemble
This specialized command is provided to dump a range of memory as machine instructions. The default memory range is the function surrounding the program counter of the selected frame. A single argument to this command is a program counter value; the function surrounding this value will be dumped. Two arguments (separated by one or more spaces) specify a range of addresses (first inclusive, second exclusive) to be dumped.
We can use disassemble
to inspect the object code
range shown in the last info line
example:
(_GDBP__) disas 0x63e4 0x6404 Dump of assembler code from 0x63e4 to 0x6404: 0x63e4 <builtin_init+5340>: ble 0x63f8 <builtin_init+5360> 0x63e8 <builtin_init+5344>: sethi %hi(0x4c00), %o0 0x63ec <builtin_init+5348>: ld [%i1+4], %o0 0x63f0 <builtin_init+5352>: b 0x63fc <builtin_init+5364> 0x63f4 <builtin_init+5356>: ld [%o0+4], %o0 0x63f8 <builtin_init+5360>: or %o0, 0x1a4, %o0 0x63fc <builtin_init+5364>: call 0x9288 <path_search> 0x6400 <builtin_init+5368>: nop End of assembler dump. (_GDBP__)
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The usual way to examine data in your program is with the print
command (abbreviated p
), or its synonym inspect
. It
evaluates and prints the value of an expression of the language your
program is written in (see section Using _GDBN__ with Different Languages).
print exp
print /f exp
exp is an expression (in the source language). By default the value of exp is printed in a format appropriate to its data type; you can choose a different format by specifying ‘/f’, where f is a letter specifying the format; see section Output formats.
print
print /f
If you omit exp, _GDBN__ displays the last value again (from the value history; see section Value History). This allows you to conveniently inspect the same value in an alternative format.
A more low-level way of examining data is with the x
command.
It examines data in memory at a specified address and prints it in a
specified format. See section Examining Memory.
If you’re interested in information about types, or about how the fields
of a struct or class are declared, use the ptype exp
command rather than print
. See section Examining the Symbol Table.
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print
and many other _GDBN__ commands accept an expression and
compute its value. Any kind of constant, variable or operator defined
by the programming language you are using is legal in an expression in
_GDBN__. This includes conditional expressions, function calls, casts
and string constants. It unfortunately does not include symbols defined
by preprocessor #define
commands.
Because C is so widespread, most of the expressions shown in examples in this manual are in C. See section Using _GDBN__ with Different Languages, for information on how to use expressions in other languages.
In this section, we discuss operators that you can use in _GDBN__ expressions regardless of your programming language.
Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer so as to examine a structure at that address in memory.
_GDBN__ supports these operators in addition to those of programming languages:
@
‘@’ is a binary operator for treating parts of memory as arrays. See section Artificial Arrays, for more information.
::
‘::’ allows you to specify a variable in terms of the file or function where it is defined. See section Program Variables.
{type} addr
Refers to an object of type type stored at address addr in memory. addr may be any expression whose value is an integer or pointer (but parentheses are required around binary operators, just as in a cast). This construct is allowed regardless of what kind of data is normally supposed to reside at addr.
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The most common kind of expression to use is the name of a variable in your program.
Variables in expressions are understood in the selected stack frame (see section Selecting a Frame); they must either be global (or static) or be visible according to the scope rules of the programming language from the point of execution in that frame. This means that in the function
foo (a) int a; { bar (a); { int b = test (); bar (b); } }
the variable a
is usable whenever the program is executing
within the function foo
, but the variable b
is visible
only while the program is executing inside the block in which b
is declared.
There is an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point is not in this file. But it is possible to have more than one such variable or function with the same name (in different source files). If that happens, referring to that name has unpredictable effects. If you wish, you can specify a variable in a particular file, using the colon-colon notation:
file::variable
Here file is the name of the source file whose variable you want.
This use of ‘::’ is very rarely in conflict with the very similar use of the same notation in C++. _GDBN__ also supports use of the C++ scope resolution operator in _GDBN__ expressions.
Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function—just after entry to the function, and just before exit. You may see this problem when you’re stepping by machine instructions. This is because on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you’re stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On function exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.
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It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program.
This can be done by constructing an artificial array with the binary operator ‘@’. The left operand of ‘@’ should be the first element of the desired array, as an individual object. The right operand should be the desired length of the array. The result is an array value whose elements are all of the type of the left argument. The first element is actually the left argument; the second element comes from bytes of memory immediately following those that hold the first element, and so on. Here is an example. If a program says
int *array = (int *) malloc (len * sizeof (int));
you can print the contents of array
with
p *array@len
The left operand of ‘@’ must reside in memory. Array values made with ‘@’ in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (see section Value History), after printing one out.)
Sometimes the artificial array mechanism isn’t quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent—for example, if you’re interested in the values
of pointers in an array. One useful work-around in this situation is to
use a convenience variable (see section Convenience Variables) as a counter in an
expression that prints the first interesting value, and then repeat that
expression via <RET>. For instance, suppose you have an array
dtab
of pointers to structures, and you’re interested in the
values of a field fv
in each structure. Here’s an example of
what you might type:
set $i = 0 p dtab[$i++]->fv <RET> <RET> …
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By default, _GDBN__ prints a value according to its data type. Sometimes this is not what you want. For example, you might want to print a number in hex, or a pointer in decimal. Or you might want to view data in memory at a certain address as a character string or as an instruction. To do these things, specify an output format when you print a value.
The simplest use of output formats is to say how to print a value
already computed. This is done by starting the arguments of the
print
command with a slash and a format letter. The format
letters supported are:
x
Regard the bits of the value as an integer, and print the integer in hexadecimal.
d
Print as integer in signed decimal.
u
Print as integer in unsigned decimal.
o
Print as integer in octal.
t
Print as integer in binary. The letter ‘t’ stands for “two”.
a
Print as an address, both absolute in hex and as an offset from the nearest preceding symbol. This format can be used to discover where (in what function) an unknown address is located:
(_GDBP__) p/a 0x54320 _0__$3 = 0x54320 <_initialize_vx+396>_1__
c
Regard as an integer and print it as a character constant.
f
Regard the bits of the value as a floating point number and print using typical floating point syntax.
For example, to print the program counter in hex (see section Registers), type
p/x $pc
Note that no space is required before the slash; this is because command names in _GDBN__ cannot contain a slash.
To reprint the last value in the value history with a different format,
you can use the print
command with just a format and no
expression. For example, ‘p/x’ reprints the last value in hex.
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x/nfu addr
x addr
x
You can use the command x
(for ‘examine’) to examine memory in
any of several formats, independently of your program’s data types.
n, f, and u are all optional parameters to specify how
much memory to display, and how to format it; addr is an
expression giving the address where you want to start displaying memory.
If you use defaults for nfu, you need not type the slash ‘/’.
Several commands set convenient defaults for addr.
n, the repeat count, is a decimal integer; the default is 1. It specifies how much memory (counting by units u) to display.
f, the display format, is one of the formats used by print
,
or ‘s’ (null-terminated string) or ‘i’ (machine instruction).
The default is ‘x’ (hexadecimal) initially, or the format from the
last time you used either x
or print
.
u, the unit size, is any of
b
Bytes.
h
Halfwords (two bytes).
w
Words (four bytes). This is the initial default.
g
Giant words (eight bytes).
Each time you specify a unit size with x
, that size becomes the
default unit the next time you use x
. (For the ‘s’ and
‘i’ formats, the unit size is ignored and is normally not written.)
addr is the address where you want _GDBN__ to begin displaying
memory. The expression need not have a pointer value (though it may);
it is always interpreted as an integer address of a byte of memory.
See section Expressions for more information on expressions. The default for
addr is usually just after the last address examined—but several
other commands also set the default address: info breakpoints
(to
the address of the last breakpoint listed), info line
(to the
starting address of a line), and print
(if you use it to display
a value from memory).
For example, ‘x/3uh 0x54320’ is a request to display three halfwords
(h
) of memory, formatted as unsigned decimal integers (‘u’),
starting at address 0x54320
. ‘x/4xw $sp’ prints the four
words (‘w’) of memory above the stack pointer (here, ‘$sp’;
see section Registers) in hexadecimal (‘x’).
Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you don’t have to remember whether unit size or format comes first; either order will work. The output specifications ‘4xw’ and ‘4wx’ mean exactly the same thing. (However, the count n must come first; ‘wx4’ will not work.)
Even though the unit size u is ignored for the formats ‘s’
and ‘i’, you might still want to use a count n; for example,
‘3i’ specifies that you want to see three machine instructions,
including any operands. The command disassemble
gives an
alternative way of inspecting machine instructions; see section Source and Machine Code.
All the defaults for the arguments to x
are designed to make it
easy to continue scanning memory with minimal specifications each time
you use x
. For example, after you’ve inspected three machine
instructions with ‘x/3i addr’, you can inspect the next seven
with just ‘x/7’. If you use <RET> to repeat the x
command,
the repeat count n is used again; the other arguments default as
for successive uses of x
.
The addresses and contents printed by the x
command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, _GDBN__ makes these values available for
subsequent use in expressions as values of the convenience variables
$_
and $__
. After an x
command, the last address
examined is available for use in expressions in the convenience variable
$_
. The contents of that address, as examined, are available in
the convenience variable $__
.
If the x
command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of output.
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If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the automatic display list so that _GDBN__ will print its value each time the program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like this:
2: foo = 38 3: bar[5] = (struct hack *) 0x3804
showing item numbers, expressions and their current values. As with
displays you request manually using x
or print
, you can
specify the output format you prefer; in fact, display
decides
whether to use print
or x
depending on how elaborate your
format specification is—it uses x
if you specify a unit size,
or one of the two formats (‘i’ and ‘s’) that are only
supported by x
; otherwise it uses print
.
display exp
Add the expression exp to the list of expressions to display each time the program stops. See section Expressions.
display
will not repeat if you press <RET> again after using it.
display/fmt exp
For fmt specifying only a display format and not a size or count, add the expression exp to the auto-display list but arranges to display it each time in the specified format fmt. See section Output formats.
display/fmt addr
For fmt ‘i’ or ‘s’, or including a unit-size or a number of units, add the expression addr as a memory address to be examined each time the program stops. Examining means in effect doing ‘x/fmt addr’. See section Examining Memory.
For example, ‘display/i $pc’ can be helpful, to see the machine instruction about to be executed each time execution stops (‘$pc’ is a common name for the program counter; see section Registers).
undisplay dnums…
delete display dnums…
Remove item numbers dnums from the list of expressions to display.
undisplay
will not repeat if you press <RET> after using it.
(Otherwise you would just get the error ‘No display number …’.)
disable display dnums…
Disable the display of item numbers dnums. A disabled display item is not printed automatically, but is not forgotten. It may be enabled again later.
enable display dnums…
Enable display of item numbers dnums. It becomes effective once again in auto display of its expression, until you specify otherwise.
display
Display the current values of the expressions on the list, just as is done when the program stops.
info display
Print the list of expressions previously set up to display automatically, each one with its item number, but without showing the values. This includes disabled expressions, which are marked as such. It also includes expressions which would not be displayed right now because they refer to automatic variables not currently available.
If a display expression refers to local variables, then it does not make
sense outside the lexical context for which it was set up. Such an
expression is disabled when execution enters a context where one of its
variables is not defined. For example, if you give the command
display last_char
while inside a function with an argument
last_char
, then this argument will be displayed while the program
continues to stop inside that function. When it stops elsewhere—where
there is no variable last_char
—display is disabled. The next time
your program stops where last_char
is meaningful, you can enable the
display expression once again.
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_GDBN__ provides the following ways to control how arrays, structures, and symbols are printed.
These settings are useful for debugging programs in any language:
set print address
set print address on
_GDBN__ will print memory addresses showing the location of stack
traces, structure values, pointer values, breakpoints, and so forth,
even when it also displays the contents of those addresses. The default
is on. For example, this is what a stack frame display looks like, with
set print address on
:
(_GDBP__) f #0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>") at input.c:530 530 if (lquote != def_lquote)
set print address off
Do not print addresses when displaying their contents. For example,
this is the same stack frame displayed with set print address off
:
(_GDBP__) set print addr off (_GDBP__) f #0 set_quotes (lq="<<", rq=">>") at input.c:530 530 if (lquote != def_lquote)
show print address
Show whether or not addresses are to be printed.
set print array
set print array on
_GDBN__ will pretty print arrays. This format is more convenient to read, but uses more space. The default is off.
set print array off.
Return to compressed format for arrays.
show print array
Show whether compressed or pretty format is selected for displaying arrays.
set print elements number-of-elements
If _GDBN__ is printing a large array, it will stop printing after it has
printed the number of elements set by the set print elements
command.
This limit also applies to the display of strings.
show print elements
Display the number of elements of a large array that _GDBN__ will print before losing patience.
set print pretty on
Cause _GDBN__ to print structures in an indented format with one member per line, like this:
$1 = { next = 0x0, flags = { sweet = 1, sour = 1 }, meat = 0x54 "Pork" }
set print pretty off
Cause _GDBN__ to print structures in a compact format, like this:
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, meat \ = 0x54 "Pork"}
This is the default format.
show print pretty
Show which format _GDBN__ will use to print structures.
set print sevenbit-strings on
Print using only seven-bit characters; if this option is set,
_GDBN__ will display any eight-bit characters (in strings or character
values) using the notation \
nnn. For example, M-a is
displayed as \341
.
set print sevenbit-strings off
Print using either seven-bit or eight-bit characters, as required. This is the default.
show print sevenbit-strings
Show whether or not _GDBN__ will print only seven-bit characters.
set print union on
Tell _GDBN__ to print unions which are contained in structures. This is the default setting.
set print union off
Tell _GDBN__ not to print unions which are contained in structures.
show print union
Ask _GDBN__ whether or not it will print unions which are contained in structures.
For example, given the declarations
typedef enum {Tree, Bug} Species; typedef enum {Big_tree, Acorn, Seedling} Tree_forms; typedef enum {Caterpillar, Cocoon, Butterfly} Bug_forms; struct thing { Species it; union { Tree_forms tree; Bug_forms bug; } form; }; struct thing foo = {Tree, {Acorn}};
with set print union on
in effect ‘p foo’ would print
$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}
and with set print union off
in effect it would print
$1 = {it = Tree, form = {...}}
These settings are of interest when debugging C++ programs:
set print demangle
set print demangle on
Print C++ names in their source form rather than in the mangled form in which they are passed to the assembler and linker for type-safe linkage. The default is on.
show print demangle
Show whether C++ names will be printed in mangled or demangled form.
set print asm-demangle
set print asm-demangle on
Print C++ names in their source form rather than their mangled form, even in assembler code printouts such as instruction disassemblies. The default is off.
show print asm-demangle
Show whether C++ names in assembly listings will be printed in mangled or demangled form.
set print object
set print object on
When displaying a pointer to an object, identify the actual (derived) type of the object rather than the declared type, using the virtual function table.
set print object off
Display only the declared type of objects, without reference to the virtual function table. This is the default setting.
show print object
Show whether actual, or declared, object types will be displayed.
set print vtbl
set print vtbl on
Pretty print C++ virtual function tables. The default is off.
set print vtbl off
Do not pretty print C++ virtual function tables.
show print vtbl
Show whether C++ virtual function tables are pretty printed, or not.
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Values printed by the print
command are saved in _GDBN__’s value
history so that you can refer to them in other expressions. Values are
kept until the symbol table is re-read or discarded (for example with
the file
or symbol-file
commands). When the symbol table
changes, the value history is discarded, since the values may contain
pointers back to the types defined in the symbol table.
The values printed are given history numbers for you to refer to them
by. These are successive integers starting with one. print
shows you
the history number assigned to a value by printing ‘$num = ’
before the value; here num is the history number.
To refer to any previous value, use ‘$’ followed by the value’s
history number. The way print
labels its output is designed to
remind you of this. Just $
refers to the most recent value in
the history, and $$
refers to the value before that.
$$n
refers to the nth value from the end; $$2
is the value just prior to $$
, $$1
is equivalent to
$$
, and $$0
is equivalent to $
.
For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure. It suffices to type
p *$
If you have a chain of structures where the component next
points
to the next one, you can print the contents of the next one with this:
p *$.next
You can print successive links in the chain by repeating this command—which you can do by just typing <RET>.
Note that the history records values, not expressions. If the value of
x
is 4 and you type these commands:
print x set x=5
then the value recorded in the value history by the print
command
remains 4 even though the value of x
has changed.
show values
Print the last ten values in the value history, with their item numbers.
This is like ‘p $$9’ repeated ten times, except that show
values
does not change the history.
show values n
Print ten history values centered on history item number n.
show values +
Print ten history values just after the values last printed. If no more values are available, produces no display.
Pressing <RET> to repeat show values n
has exactly the
same effect as ‘show values +’.
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_GDBN__ provides convenience variables that you can use within _GDBN__ to hold on to a value and refer to it later. These variables exist entirely within _GDBN__; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That’s why you can use them freely.
Convenience variables are prefixed with ‘$’. Any name preceded by ‘$’ can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see section Registers). (Value history references, in contrast, are numbers preceded by ‘$’. See section Value History.)
You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. Example:
set $foo = *object_ptr
would save in $foo
the value contained in the object pointed to by
object_ptr
.
Using a convenience variable for the first time creates it; but its value
is void
until you assign a new value. You can alter the value with
another assignment at any time.
Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value.
show convenience
Print a list of convenience variables used so far, and their values.
Abbreviated show con
.
One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For example, to print a field from successive elements of an array of structures:
_0__
set $i = 0 print bar[$i++]->contents … repeat that command by typing <RET>. _1__
Some convenience variables are created automatically by _GDBN__ and given values likely to be useful.
$_
The variable $_
is automatically set by the x
command to
the last address examined (see section Examining Memory). Other commands which
provide a default address for x
to examine also set $_
to that address; these commands include info line
and info
breakpoint
. The type of $_
is void *
except when set by the
x
command, in which case it is a pointer to the type of $__
.
$__
The variable $__
is automatically set by the x
command
to the value found in the last address examined. Its type is chosen
to match the format in which the data was printed.
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You can refer to machine register contents, in expressions, as variables
with names starting with ‘$’. The names of registers are different
for each machine; use info registers
to see the names used on
your machine.
info registers
Print the names and values of all registers except floating-point registers (in the selected stack frame).
info all-registers
Print the names and values of all registers, including floating-point registers.
info registers regname
Print the relativized value of register regname. regname may be any register name valid on the machine you are using, with or without the initial ‘$’.
_GDBN__ has four “standard” register names that are available (in
expressions) on most machines—whenever they don’t conflict with an
architecture’s canonical mnemonics for registers. The register names
$pc
and $sp
are used for the program counter register and
the stack pointer. $fp
is used for a register that contains a
pointer to the current stack frame, and $ps
is used for a
register that contains the processor status. For example,
you could print the program counter in hex with
p/x $pc
or print the instruction to be executed next with
x/i $pc
or add four to the stack pointer (1) with
set $sp += 4
Whenever possible, these four standard register names are available on
your machine even though the machine has different canonical mnemonics,
so long as there is no conflict. The info registers
command
shows the canonical names. For example, on the SPARC, info
registers
displays the processor status register as $psr
but you
can also refer to it as $ps
.
_GDBN__ always considers the contents of an ordinary register as an integer when the register is examined in this way. Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values. There is no way to refer to the contents of an ordinary register as floating point value (although you can print it as a floating point value with ‘print/f $regname’).
Some registers have distinct “raw” and “virtual” data formats. This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees. For example, the registers of the 68881 floating point
coprocessor are always saved in “extended” (raw) format, but all C
programs expect to work with “double” (virtual) format. In such
cases, _GDBN__ normally works with the virtual format only (the format that
makes sense for your program), but the info registers
command
prints the data in both formats.
Normally, register values are relative to the selected stack frame (see section Selecting a Frame). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with ‘frame 0’).
However, _GDBN__ must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if _GDBN__ is unable to locate the saved registers, the selected stack frame will make no difference.
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Depending on the host machine architecture, _GDBN__ may be able to give you more information about the status of the floating point hardware.
info float
If available, provides hardware-dependent information about the floating point unit. The exact contents and layout vary depending on the floating point chip.
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Although programming languages generally have common aspects, they are
rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer p
is accomplished by *p
, but in
Modula-2, it is accomplished by p^
. Values can also be
represented (and displayed) differently. Hex numbers in C are written
like ‘0x1ae’, while in Modula-2 they appear as ‘1AEH’.
Language-specific information is built into _GDBN__ for some languages, allowing you to express operations like the above in the program’s native language, and allowing _GDBN__ to output values in a manner consistent with the syntax of the program’s native language. The language you use to build expressions, called the working language, can be selected manually, or _GDBN__ can set it automatically.
9.1 Switching between source languages | ||
9.2 Displaying the language | ||
9.3 Type and range Checking | Type and Range checks | |
9.4 Supported Languages | Supported languages |
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There are two ways to control the working language—either have _GDBN__
set it automatically, or select it manually yourself. You can use the
set language
command for either purpose. On startup, _GDBN__
defaults to setting the language automatically.
9.1.1 Setting the working language | Setting the working language manually | |
9.1.2 Having _GDBN__ infer the source language |
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To set the language, issue the command ‘set language lang’,
where lang is the name of a language: c
or modula-2
.
For a list of the supported languages, type ‘set language’.
Setting the language manually prevents _GDBN__ from updating the working language automatically. This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages—but means different things. For instance, if the current source file were written in C, and _GDBN__ was parsing Modula-2, a command such as:
print a = b + c
might not have the effect you intended. In C, this means to add
b
and c
and place the result in a
. The result
printed would be the value of a
. In Modula-2, this means to compare
a
to the result of b+c
, yielding a BOOLEAN
value.
If you allow _GDBN__ to set the language automatically, then you can count on expressions evaluating the same way in your debugging session and in your program.
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To have _GDBN__ set the working language automatically, use ‘set language local’ or ‘set language auto’. _GDBN__ then infers the language that a program was written in by looking at the name of its source files, and examining their extensions:
Modula-2 source file
C or C++ source file.
This information is recorded for each function or procedure in a source file. When your program stops in a frame (usually by encountering a breakpoint), _GDBN__ sets the working language to the language recorded for the function in that frame. If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and _GDBN__ issues a warning.
This may not seem necessary for most programs, which are written entirely in one source language. However, program modules and libraries written in one source language can be used by a main program written in a different source language. Using ‘set language auto’ in this case frees you from having to set the working language manually.
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The following commands will help you find out which language is the working language, and also what language source files were written in.
show language
Display the current working language. This is the
language you can use with commands such as print
to
build and compute expressions that may involve variables in the program.
info frame
Among the other information listed here (see section Information about a Frame) is the source language for this frame. This is the language that will become the working language if you ever use an identifier that is in this frame.
info source
Among the other information listed here (see section Examining the Symbol Table) is the source language of this source file.
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Warning: In this release, the _GDBN__ commands for type and range checking are included, but they do not yet have any effect. This section documents the intended facilities.
Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks. These include checking the type of arguments to functions and operators, and making sure mathematical overflows are caught at run time. Checks such as these help to ensure a program’s correctness once it has been compiled by eliminating type mismatches, and providing active checks for range errors when the program is running.
_GDBN__ can check for conditions like the above if you wish.
Although _GDBN__ will not check the statements in your program, it
can check expressions entered directly into _GDBN__ for evaluation via
the print
command, for example. As with the working language,
_GDBN__ can also decide whether or not to check automatically based on
the source language of the program being debugged.
See section Supported Languages, for the default settings
of supported languages.
9.3.1 An overview of type checking | ||
9.3.2 An overview of Range Checking | An overview of range checking |
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Some languages, such as Modula-2, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs. These checks prevent type mismatch errors from ever causing any run-time problems. For example,
1 + 2 ⇒ 3 error--> 1 + 2.3
The second example fails because the CARDINAL
1 is not
type-compatible with the REAL
2.3.
For expressions you use in _GDBN__ commands, you can tell the _GDBN__ type checker to skip checking; to treat any mismatches as errors and abandon the expression; or only issue warnings when type mismatches occur, but evaluate the expression anyway. When you choose the last of these, _GDBN__ evaluates expressions like the second example above, but also issues a warning.
Even though you may turn type checking off, other type-based reasons may
prevent _GDBN__ from evaluating an expression. For instance, _GDBN__ does not
know how to add an int
and a struct foo
. These particular
type errors have nothing to do with the language in use, and usually
arise from expressions, such as the one described above, which make
little sense to evaluate anyway.
Each language defines to what degree it is strict about type. For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers. In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators. See section Supported Languages, for futher details on specific languages.
_GDBN__ provides some additional commands for controlling the type checker:
set check type auto
Set type checking on or off based on the current working language. See section Supported Languages, for the default settings for each language.
set check type on
set check type off
Set type checking on or off, overriding the default setting for the current working language. Issue a warning if the setting does not match the language’s default. If any type mismatches occur in evaluating an expression while typechecking is on, _GDBN__ prints a message and aborts evaluation of the expression.
set check type warn
Cause the type checker to issue warnings, but to always attempt to evaluate the expression. Evaluating the expression may still be impossible for other reasons. For example, _GDBN__ cannot add numbers and structures.
show type
Show the current setting of the type checker, and whether or not _GDBN__ is setting it automatically.
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In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks. Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array.
For expressions you use in _GDBN__ commands, you can tell _GDBN__ to ignore range errors; to always treat them as errors and abandon the expression; or to issue warnings when a range error occurs but evaluate the expression anyway.
A range error can result from numerical overflow, from exceeding an array index bound, or when you type in a constant that is not a member of any type. Some languages, however, do not treat overflows as an error. In many implementations of C, mathematical overflow causes the result to “wrap around” to lower values—for example, if m is the largest integer value, and s is the smallest, then
m + 1 ⇒ s
This, too, is specific to individual languages, and in some cases specific to individual compilers or machines. See section Supported Languages, for further details on specific languages.
_GDBN__ provides some additional commands for controlling the range checker:
set check range auto
Set range checking on or off based on the current working language. See section Supported Languages, for the default settings for each language.
set check range on
set check range off
Set range checking on or off, overriding the default setting for the current working language. A warning is issued if the setting does not match the language’s default. If a range error occurs, then a message is printed and evaluation of the expression is aborted.
set check range warn
Output messages when the _GDBN__ range checker detects a range error, but attempt to evaluate the expression anyway. Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many UNIX systems).
show range
Show the current setting of the range checker, and whether or not it is being set automatically by _GDBN__.
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_GDBN__ _GDB_VN__ supports C, C++, and Modula-2. The syntax for C and C++ is
so closely related that _GDBN__ does not distinguish the two. Some
_GDBN__ features may be used in expressions regardless of the language
you use: the _GDBN__ @
and ::
operators, and the
‘{type}addr’ construct (see section Expressions) can be used with the constructs of
any of the supported languages.
The following sections detail to what degree each of these source languages is supported by _GDBN__. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the _GDBN__ expression parser will accept, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial.
9.4.1 C and C++ | ||
9.4.2 Modula-2 |
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Since C and C++ are so closely related, _GDBN__ does not distinguish between them when interpreting the expressions recognized in _GDBN__ commands.
The C++ debugging facilities are jointly implemented by the GNU C++
compiler and _GDBN__. Therefore, to debug your C++ code effectively,
you must compile your C++ programs with the GNU C++ compiler,
g++
.
9.4.1.1 C and C++ Operators | ||
9.4.1.2 C and C++ Constants | ||
9.4.1.3 C++ Expressions | ||
9.4.1.4 C and C++ Defaults | Default settings for C and C++ | |
9.4.1.5 C and C++ Type and Range Checks | ||
9.4.1.6 _GDBN__ and C | ||
9.4.1.7 _GDBN__ Commands for C++ | Special features for C++ |
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Operators must be defined on values of specific types. For instance,
+
is defined on numbers, but not on structures. Operators are
often defined on groups of types. For the purposes of C and C++, the
following definitions hold:
int
with any of its storage-class
specifiers, char
, and enum
s.
float
and double
.
(type
*)
.
The following operators are supported. They are listed here in order of increasing precedence:
_0__
,
The comma or sequencing operator. Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated.
=
Assignment. The value of an assignment expression is the value assigned. Defined on scalar types.
op=
Used in an expression of the form a op=
b, and
translated to a =
a op b. op=
and
=
have the same precendence. op is any one of the
operators |
, ^
, &
, <<
, >>
, +
,
-
, *
, /
, %
.
?:
The ternary operator. a ? b : c
can be thought
of as: if a then b else c. a should be of an
integral type.
||
Logical OR. Defined on integral types.
&&
Logical AND. Defined on integral types.
|
Bitwise OR. Defined on integral types.
^
Bitwise exclusive-OR. Defined on integral types.
&
Bitwise AND. Defined on integral types.
==, !=
Equality and inequality. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.
<, >, <=, >=
Less than, greater than, less than or equal, greater than or equal. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.
<<, >>
left shift, and right shift. Defined on integral types.
@
The _GDBN__ “artificial array” operator (see section Expressions).
+, -
Addition and subtraction. Defined on integral types, floating-point types and pointer types.
*, /, %
Multiplication, division, and modulus. Multiplication and division are defined on integral and floating-point types. Modulus is defined on integral types.
++, --
Increment and decrement. When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable’s value is used before the operation takes place.
*
Pointer dereferencing. Defined on pointer types. Same precedence as
++
.
&
Address operator. Defined on variables. Same precedence as ++
.
-
Negative. Defined on integral and floating-point types. Same
precedence as ++
.
!
Logical negation. Defined on integral types. Same precedence as
++
.
~
Bitwise complement operator. Defined on integral types. Same precedence as
++
.
., ->
Structure member, and pointer-to-structure member. For convenience,
_GDBN__ regards the two as equivalent, choosing whether to dereference a
pointer based on the stored type information.
Defined on struct
s and union
s.
[]
Array indexing. a[i]
is defined as
*(a+i)
. Same precedence as ->
.
()
Function parameter list. Same precedence as ->
.
::
C++ scope resolution operator. Defined on
struct
, union
, and class
types.
::
The _GDBN__ scope operator (see section Expressions). Same precedence as
::
, above. _1__
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_GDBN__ allows you to express the constants of C and C++ in the following ways:
long
value.
'
), or a number—the ordinal value of the corresponding character
(usually its ASCII value). Within quotes, the single character may
be represented by a letter or by escape sequences, which are of
the form ‘\nnn’, where nnn is the octal representation
of the character’s ordinal value; or of the form ‘\x’, where
‘x’ is a predefined special character—for example,
‘\n’ for newline.
"
).
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_GDBN__’s expression handling has the following extensions to interpret a significant subset of C++ expressions:
count = aml->GetOriginal(x, y)
this
following the same rules as C++.
In the parameter list shown when _GDBN__ displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you’ve specified ‘set print address off’.
::
—your
expressions can use it just as expressions in your program do. Since
one scope may be defined in another, you can use ::
repeatedly if
necessary, for example in an expression like
‘scope1::scope2::name’. _GDBN__ also allows
resolving name scope by reference to source files, in both C and C++
debugging; see section Program Variables.
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If you allow _GDBN__ to set type and range checking automatically, they
both default to off
whenever the working language changes to
C/C++. This happens regardless of whether you, or _GDBN__,
selected the working language.
If you allow _GDBN__ to set the language automatically, it sets the working language to C/C++ on entering code compiled from a source file whose name ends with ‘.c’ or ‘.cc’. See section Having _GDBN__ infer the source language, for further details.
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Warning: in this release, _GDBN__ does not yet perform type or range checking.
By default, when _GDBN__ parses C or C++ expressions, type checking is not used. However, if you turn type checking on, _GDBN__ will consider two variables type equivalent if:
typedef
.
Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array.
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The set print union
and show print union
commands apply to
the union
type. When set to ‘on’, any union
that is
inside a struct
or class
will also be printed.
Otherwise, it will appear as ‘{...}’.
The @
operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function. (see section Expressions)
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Some _GDBN__ commands are particularly useful with C++, and some are designed specifically for use with C++. Here is a summary:
breakpoint menus
When you want a breakpoint in a function whose name is overloaded, _GDBN__’s breakpoint menus help you specify which function definition you want. See section Breakpoint Menus.
rbreak regex
Setting breakpoints using regular expressions is helpful for setting breakpoints on overloaded functions that are not members of any special classes. See section Setting Breakpoints.
catch exceptions
info catch
Debug C++ exception handling using these commands. See section Breakpoints and Exceptions.
ptype typename
Print inheritance relationships as well as other information for type typename. See section Examining the Symbol Table.
set print demangle
show print demangle
set print asm-demangle
show print asm-demangle
Control whether C++ symbols display in their source form, both when displaying code as C++ source and when displaying disassemblies. See section Print Settings.
set print object
show print object
Choose whether to print derived (actual) or declared types of objects. See section Print Settings.
set print vtbl
show print vtbl
Control the format for printing virtual function tables. See section Print Settings.
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The extensions made to _GDBN__ to support Modula-2 support output from the GNU Modula-2 compiler (which is currently being developed). Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them will most likely result in an error as _GDBN__ reads in the executable’s symbol table.
9.4.2.1 Operators | Built-in operators | |
9.4.2.2 Built-in Functions and Procedures | ||
9.4.2.3 Constants | Modula-2 Constants | |
9.4.2.4 Modula-2 Defaults | Default settings for Modula-2 | |
9.4.2.5 Deviations from Standard Modula-2 | Deviations from standard Modula-2 | |
9.4.2.6 Modula-2 Type and Range Checks | ||
9.4.2.7 The scope operators :: and . | ||
9.4.2.8 _GDBN__ and Modula-2 |
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Operators must be defined on values of specific types. For instance,
+
is defined on numbers, but not on structures. Operators are
often defined on groups of types. For the purposes of Modula-2, the
following definitions hold:
INTEGER
, CARDINAL
, and
their subranges.
CHAR
and its subranges.
REAL
.
POINTER TO
type
.
SET
s and BITSET
s.
BOOLEAN
.
The following operators are supported, and appear in order of increasing precedence:
_0__
,
Function argument or array index separator.
:=
Assignment. The value of var :=
value is
value.
<, >
Less than, greater than on integral, floating-point, or enumerated types.
<=, >=
Less than, greater than, less than or equal to, greater than or equal to
on integral, floating-point and enumerated types, or set inclusion on
set types. Same precedence as <
.
=, <>, #
Equality and two ways of expressing inequality, valid on scalar types.
Same precedence as <
. In _GDBN__ scripts, only <>
is
available for inequality, since #
conflicts with the script
comment character.
IN
Set membership. Defined on set types and the types of their members.
Same precedence as <
.
OR
Boolean disjunction. Defined on boolean types.
AND, &
Boolean conjuction. Defined on boolean types.
@
The _GDBN__ “artificial array” operator (see section Expressions).
+, -
Addition and subtraction on integral and floating-point types, or union and difference on set types.
*
Multiplication on integral and floating-point types, or set intersection on set types.
/
Division on floating-point types, or symmetric set difference on set
types. Same precedence as *
.
DIV, MOD
Integer division and remainder. Defined on integral types. Same
precedence as *
.
-
Negative. Defined on INTEGER
s and REAL
s.
^
Pointer dereferencing. Defined on pointer types.
NOT
Boolean negation. Defined on boolean types. Same precedence as
^
.
.
RECORD
field selector. Defined on RECORD
s. Same
precedence as ^
.
[]
Array indexing. Defined on ARRAY
s. Same precedence as ^
.
()
Procedure argument list. Defined on PROCEDURE
s. Same precedence
as ^
.
::, .
_GDBN__ and Modula-2 scope operators.
Warning: Sets and their operations are not yet supported, so _GDBN__ will treat the use of the operator
IN
, or the use of operators+
,-
,*
,/
,=
, ,<>
,#
,<=
, and>=
on sets as an error.
_1__
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Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:
represents an ARRAY
variable.
represents a CHAR
constant or variable.
represents a variable or constant of integral type.
represents an identifier that belongs to a set. Generally used in the
same function with the metavariable s. The type of s should
be SET OF mtype
(where mtype is the type of m.
represents a variable or constant of integral or floating-point type.
represents a variable or constant of floating-point type.
represents a type.
represents a variable.
represents a variable or constant of one of many types. See the explanation of the function for details.
All Modula-2 built-in procedures also return a result, described below.
ABS(n)
Returns the absolute value of n.
CAP(c)
If c is a lower case letter, it returns its upper case equivalent, otherwise it returns its argument
CHR(i)
Returns the character whose ordinal value is i.
DEC(v)
Decrements the value in the variable v. Returns the new value.
DEC(v,i)
Decrements the value in the variable v by i. Returns the new value.
EXCL(m,s)
Removes the element m from the set s. Returns the new set.
FLOAT(i)
Returns the floating point equivalent of the integer i.
HIGH(a)
Returns the index of the last member of a.
INC(v)
Increments the value in the variable v. Returns the new value.
INC(v,i)
Increments the value in the variable v by i. Returns the new value.
INCL(m,s)
Adds the element m to the set s if it is not already there. Returns the new set.
MAX(t)
Returns the maximum value of the type t.
MIN(t)
Returns the minimum value of the type t.
ODD(i)
Returns boolean TRUE if i is an odd number.
ORD(x)
Returns the ordinal value of its argument. For example, the ordinal value of a character is its ASCII value (on machines supporting the ASCII character set). x must be of an ordered type, which include integral, character and enumerated types.
SIZE(x)
Returns the size of its argument. x can be a variable or a type.
TRUNC(r)
Returns the integral part of r.
VAL(t,i)
Returns the member of the type t whose ordinal value is i.
Warning: Sets and their operations are not yet supported, so _GDBN__ will treat the use of procedures
INCL
andEXCL
as an error.
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_GDBN__ allows you to express the constants of Modula-2 in the following ways:
'
) or double ("
). They may
also be expressed by their ordinal value (their ASCII value, usually)
followed by a ‘C’.
'
) or double ("
). Escape
sequences in the style of C are also allowed. See section C and C++ Constants, for a
brief explanation of escape sequences.
TRUE
and
FALSE
.
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If type and range checking are set automatically by _GDBN__, they
both default to on
whenever the working language changes to
Modula-2. This happens regardless of whether you, or _GDBN__,
selected the working language.
If you allow _GDBN__ to set the language automatically, then entering code compiled from a file whose name ends with ‘.mod’ will set the working language to Modula-2. See section Having _GDBN__ set the language automatically, for further details.
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A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:
:=
) returns the value of its right-hand
argument.
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Warning: in this release, _GDBN__ does not yet perform type or range checking.
_GDBN__ considers two Modula-2 variables type equivalent if:
TYPE
t1 = t2
statement
As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error.
Range checking is done on all mathematical operations, assignment, array index bounds, and all builtin functions and procedures.
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::
and .
There are a few subtle differences between the Modula-2 scope operator
(.
) and the _GDBN__ scope operator (::
). The two have
similar syntax:
module . id scope :: id
where scope is the name of a module or a procedure, module the name of a module, and id is any delcared identifier within the program, except another module.
Using the ::
operator makes _GDBN__ search the scope
specified by scope for the identifier id. If it is not
found in the specified scope, then _GDBN__ will search all scopes
enclosing the one specified by scope.
Using the .
operator makes _GDBN__ search the current scope for
the identifier specified by id that was imported from the
definition module specified by module. With this operator, it is
an error if the identifier id was not imported from definition
module module, or if id is not an identifier in
module.
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Some _GDBN__ commands have little use when debugging Modula-2 programs.
Five subcommands of set print
and show print
apply
specifically to C and C++: ‘vtbl’, ‘demangle’,
‘asm-demangle’, ‘object’, and ‘union’. The first four
apply to C++, and the last to C’s union
type, which has no direct
analogue in Modula-2.
The @
operator (see section Expressions), while available
while using any language, is not useful with Modula-2. Its
intent is to aid the debugging of dynamic arrays, which cannot be
created in Modula-2 as they can in C or C++. However, because an
address can be specified by an integral constant, the construct
‘{type}adrexp’ is still useful. (see section Expressions)
_0__
In _GDBN__ scripts, the Modula-2 inequality operator #
is
interpreted as the beginning of a comment. Use <>
instead.
_1__
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The commands described in this section allow you to inquire about the symbols (names of variables, functions and types) defined in your program. This information is inherent in the text of your program and does not change as the program executes. _GDBN__ finds it in your program’s symbol table, in the file indicated when you started _GDBN__ (see section Choosing Files), or by one of the file-management commands (see section Commands to Specify Files).
info address symbol
Describe where the data for symbol is stored. For a register variable, this says which register it is kept in. For a non-register local variable, this prints the stack-frame offset at which the variable is always stored.
Note the contrast with ‘print &symbol’, which does not work at all for a register variables, and for a stack local variable prints the exact address of the current instantiation of the variable.
whatis exp
Print the data type of expression exp. exp is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place. See section Expressions.
whatis
Print the data type of $
, the last value in the value history.
ptype typename
Print a description of data type typename. typename may be the name of a type, or for C code it may have the form ‘struct struct-tag’, ‘union union-tag’ or ‘enum enum-tag’.
ptype exp
ptype
Print a description of the type of expression exp. ptype
differs from whatis
by printing a detailed description, instead of just
the name of the type. For example, if your program declares a variable
as
struct complex {double real; double imag;} v;
compare the output of the two commands:
(_GDBP__) whatis v type = struct complex (_GDBP__) ptype v type = struct complex { double real; double imag; }
As with whatis
, using ptype
without an argument refers to
the type of $
, the last value in the value history.
info types regexp
info types
Print a brief description of all types whose name matches regexp
(or all types in your program, if you supply no argument). Each
complete typename is matched as though it were a complete line; thus,
‘i type value’ gives information on all types in your program whose
name includes the string value
, but ‘i type ^value$’ gives
information only on types whose complete name is value
.
This command differs from ptype
in two ways: first, like
whatis
, it does not print a detailed description; second, it
lists all source files where a type is defined.
info source
Show the name of the current source file—that is, the source file for the function containing the current point of execution—and the language it was written in.
info sources
Print the names of all source files in the program for which there is debugging information, organized into two lists: files whose symbols have already been read, and files whose symbols will be read when needed.
info functions
Print the names and data types of all defined functions.
info functions regexp
Print the names and data types of all defined functions
whose names contain a match for regular expression regexp.
Thus, ‘info fun step’ finds all functions whose names
include step
; ‘info fun ^step’ finds those whose names
start with step
.
info variables
Print the names and data types of all variables that are declared outside of functions (i.e., excluding local variables).
info variables regexp
Print the names and data types of all variables (except for local variables) whose names contain a match for regular expression regexp.
printsyms filename
printpsyms filename
Write a dump of debugging symbol data into the file filename.
These commands are used to debug the _GDBN__ symbol-reading code. Only
symbols with debugging data are included. If you use printsyms
,
_GDBN__ includes all the symbols for which it has already collected full
details: that is, filename reflects symbols for only those files
whose symbols _GDBN__ has read. You can use the command info
sources
to find out which files these are. If you use
printpsyms
, the dump also shows information about symbols that
_GDBN__ only knows partially—that is, symbols defined in files that
_GDBN__ has skimmed, but not yet read completely. The description of
symbol-file
describes how _GDBN__ reads symbols; both commands
are described under Commands to Specify Files.
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Once you think you have found an error in the program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run. You can find the answer by experiment, using the _GDBN__ features for altering execution of the program.
For example, you can store new values into variables or memory locations, give the program a signal, restart it at a different address, or even return prematurely from a function to its caller.
11.1 Assignment to Variables | ||
11.2 Continuing at a Different Address | ||
11.3 Giving the Program a Signal | ||
11.4 Returning from a Function | ||
11.5 Calling your Program’s Functions | ||
11.6 Patching your Program |
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To alter the value of a variable, evaluate an assignment expression. See section Expressions. For example,
print x=4
would store the value 4 into the variable x
, and then print the
value of the assignment expression (which is 4). See section Using _GDBN__ with Different Languages, for
more information on operators in supported languages.
If you are not interested in seeing the value of the assignment, use the
set
command instead of the print
command. set
is
really the same as print
except that the expression’s value is not
printed and is not put in the value history (see section Value History). The
expression is evaluated only for its effects.
If the beginning of the argument string of the set
command
appears identical to a set
subcommand, use the set
variable
command instead of just set
. This command is identical
to set
except for its lack of subcommands. For example, a
program might well have a variable width
—which leads to
an error if we try to set a new value with just ‘set width=13’, as
we might if set width
didn’t happen to be a _GDBN__ command:
(_GDBP__) whatis width type = double (_GDBP__) p width $4 = 13 (_GDBP__) set width=47 Invalid syntax in expression.
The invalid expression, of course, is ‘=47’. What we can do in
order to actually set our program’s variable width
is
(_GDBP__) set var width=47
_GDBN__ allows more implicit conversions in assignments than C does; you can freely store an integer value into a pointer variable or vice versa, and any structure can be converted to any other structure that is the same length or shorter.
To store values into arbitrary places in memory, use the ‘{…}’
construct to generate a value of specified type at a specified address
(see section Expressions). For example, {int}0x83040
refers
to memory location 0x83040
as an integer (which implies a certain size
and representation in memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.
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Ordinarily, when you continue the program, you do so at the place where
it stopped, with the continue
command. You can instead continue at
an address of your own choosing, with the following commands:
jump linespec
Resume execution at line linespec. Execution will stop immediately if there is a breakpoint there. See section Printing Source Lines for a description of the different forms of linespec.
The jump
command does not change the current stack frame, or
the stack pointer, or the contents of any memory location or any
register other than the program counter. If line linespec is in
a different function from the one currently executing, the results may
be bizarre if the two functions expect different patterns of arguments or
of local variables. For this reason, the jump
command requests
confirmation if the specified line is not in the function currently
executing. However, even bizarre results are predictable if you are
well acquainted with the machine-language code of the program.
jump *address
Resume execution at the instruction at address address.
You can get much the same effect as the jump
command by storing a
new value into the register $pc
. The difference is that this
does not start the program running; it only changes the address where it
will run when it is continued. For example,
set $pc = 0x485
causes the next continue
command or stepping command to execute at
address 0x485, rather than at the address where the program stopped.
See section Continuing and Stepping.
The most common occasion to use the jump
command is to back up,
perhaps with more breakpoints set, over a portion of a program that has
already executed, in order to examine its execution in more detail.
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signal signalnum
Resume execution where the program stopped, but give it immediately the signal number signalnum.
Alternatively, if signalnum is zero, continue execution without
giving a signal. This is useful when the program stopped on account of
a signal and would ordinary see the signal when resumed with the
continue
command; ‘signal 0’ causes it to resume without a
signal.
signal
does not repeat when you press <RET> a second time
after executing the command.
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return
return expression
You can cancel execution of a function call with the return
command. If you give an
expression argument, its value is used as the function’s return
value.
When you use return
, _GDBN__ discards the selected stack frame
(and all frames within it). You can think of this as making the
discarded frame return prematurely. If you wish to specify a value to
be returned, give that value as the argument to return
.
This pops the selected stack frame (see section Selecting a Frame), and any other frames inside of it, leaving its caller as the innermost remaining frame. That frame becomes selected. The specified value is stored in the registers used for returning values of functions.
The return
command does not resume execution; it leaves the
program stopped in the state that would exist if the function had just
returned. In contrast, the finish
command
(see section Continuing and Stepping)
resumes execution until the selected stack frame returns naturally.
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call expr
Evaluate the expression expr without displaying void
returned values.
You can use this variant of the print
command if you want to
execute a function from your program, but without cluttering the output
with void
returned values. The result is printed and saved in
the value history, if it is not void.
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By default, _GDBN__ opens the file containing your program’s executable code (or the corefile) read-only. This prevents accidental alterations to machine code; but it also prevents you from intentionally patching your program’s binary.
If you’d like to be able to patch the binary, you can specify that
explicitly with the set write
command. For example, you might
want to turn on internal debugging flags, or even to make emergency
repairs.
set write on
set write off
If you specify ‘set write on’, _GDBN__ will open executable and core files for both reading and writing; if you specify ‘set write off’ (the default), _GDBN__ will open them read-only.
If you’ve already loaded a file, you must load it
again (using the exec-file
or core-file
command) after
changing set write
, for your new setting to take effect.
show write
Display whether executable files and core files will be opened for writing as well as reading.
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12.1 Commands to Specify Files | ||
12.2 Errors Reading Symbol Files |
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_GDBN__ needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start the program. To debug a core dump of a previous run, _GDBN__ must be told the file name of the core dump.
The usual way to specify the executable and core dump file names is with the command arguments given when you start _GDBN__, as discussed in see section Getting In and Out of _GDBN__.
Occasionally it is necessary to change to a different file during a _GDBN__ session. Or you may run _GDBN__ and forget to specify the files you want to use. In these situations the _GDBN__ commands to specify new files are useful.
file filename
Use filename as the program to be debugged. It is read for its
symbols and for the contents of pure memory. It is also the program
executed when you use the run
command. If you do not specify a
directory and the file is not found in _GDBN__’s working directory,
_GDBN__ uses the environment variable PATH
as a list of
directories to search, just as the shell does when looking for a program
to run. You can change the value of this variable, for both _GDBN__ and
your program, using the path
command.
file
file
with no argument makes _GDBN__ discard any information it
has on both executable file and the symbol table.
exec-file [ filename ]
Specify that the program to be run (but not the symbol table) is found
in filename. _GDBN__ will search the environment variable PATH
if necessary to locate the program. Omitting filename means to
discard information on the executable file.
symbol-file [ filename ]
Read symbol table information from file filename. PATH
is
searched when necessary. Use the file
command to get both symbol
table and program to run from the same file.
symbol-file
with no argument clears out _GDBN__’s information on your
program’s symbol table.
The symbol-file
command causes _GDBN__ to forget the contents of its
convenience variables, the value history, and all breakpoints and
auto-display expressions. This is because they may contain pointers to
the internal data recording symbols and data types, which are part of
the old symbol table data being discarded inside _GDBN__.
symbol-file
will not repeat if you press <RET> again after
executing it once.
On some kinds of object files, the symbol-file
command does not
actually read the symbol table in full right away. Instead, it scans
the symbol table quickly to find which source files and which symbols
are present. The details are read later, one source file at a time,
when they are needed.
The purpose of this two-stage reading strategy is to make _GDBN__ start up
faster. For the most part, it is invisible except for occasional pauses
while the symbol table details for a particular source file are being
read. (The set verbose
command can turn these pauses into
messages if desired. See section Optional Warnings and Messages).
When the symbol table is stored in COFF format, symbol-file
does
read the symbol table data in full right away. We haven’t implemented
the two-stage strategy for COFF yet.
When _GDBN__ is configured for a particular environment, it will
understand debugging information in whatever format is the standard
generated for that environment; you may use either a GNU compiler, or
other compilers that adhere to the local conventions. Best results are
usually obtained from GNU compilers; for example, using _GCC__
you can generate debugging information for optimized code.
core-file [ filename ]
Specify the whereabouts of a core dump file to be used as the “contents of memory”. Traditionally, core files contain only some parts of the address space of the process that generated them; _GDBN__ can access the executable file itself for other parts.
core-file
with no argument specifies that no core file is
to be used.
Note that the core file is ignored when your program is actually running
under _GDBN__. So, if you have been running the program and you wish to
debug a core file instead, you must kill the subprocess in which the
program is running. To do this, use the kill
command
(see section Killing the Child Process).
load filename
_if__(_GENERIC__)
Depending on what remote debugging facilities are configured into
_GDBN__, the load
command may be available. Where it exists, it
is meant to make filename (an executable) available for debugging
on the remote system—by downloading, or dynamic linking, for example.
load
also records filename’s symbol table in _GDBN__, like
the add-symbol-file
command.
If load
is not available on your _GDBN__, attempting to execute
it gets the error message “You can't do that when your target is
…
”
_fi__(_GENERIC__)
_if__(_VXWORKS__)
On VxWorks, load
will dynamically link filename on the
current target system as well as adding its symbols in _GDBN__.
_fi__(_VXWORKS__)
_if__(_I960__)
With the Nindy interface to an Intel 960 board, load
will
download filename to the 960 as well as adding its symbols in
_GDBN__.
_fi__(_I960__)
load
will not repeat if you press <RET> again after using it.
add-symbol-file filename address
The add-symbol-file
command reads additional symbol table information
from the file filename. You would use this command when filename
has been dynamically loaded (by some other means) into the program that
is running. address should be the memory address at which the
file has been loaded; _GDBN__ cannot figure this out for itself.
The symbol table of the file filename is added to the symbol table
originally read with the symbol-file
command. You can use the
add-symbol-file
command any number of times; the new symbol data thus
read keeps adding to the old. To discard all old symbol data instead,
use the symbol-file
command.
add-symbol-file
will not repeat if you press <RET> after using it.
info files
info target
info files
and info target
are synonymous; both print the
current targets (see section Specifying a Debugging Target), including the names of the executable
and core dump files currently in use by _GDBN__, and the files from
which symbols were loaded. The command help targets
lists all
possible targets rather than current ones.
All file-specifying commands allow both absolute and relative file names as arguments. _GDBN__ always converts the file name to an absolute path name and remembers it that way.
_GDBN__ supports the SunOS shared library format. _GDBN__ automatically
loads symbol definitions from shared libraries when you use the
run
command, or when you examine a core file. (Before you issue
the run
command, _GDBN__ won’t understand references to a
function in a shared library, however—unless you’re debugging a core
file).
info share
info sharedlibrary
Print the names of the shared libraries which are currently loaded.
sharedlibrary regex
share regex
This is an obsolescent command; you can use it to explicitly
load shared object library symbols for files matching a UNIX regular
expression, but as with files loaded automatically, it will only load
shared libraries required by your program for a core file or after
typing run
. If regex is omitted all shared libraries
required by your program are loaded.
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While reading a symbol file, _GDBN__ will occasionally encounter
problems, such as symbol types it does not recognize, or known bugs in
compiler output. By default, _GDBN__ does not notify you of such
problems, since they’re relatively common and primarily of interest to
people debugging compilers. If you are interested in seeing information
about ill-constructed symbol tables, you can either ask _GDBN__ to print
only one message about each such type of problem, no matter how many
times the problem occurs; or you can ask _GDBN__ to print more messages,
to see how many times the problems occur, with the set complaints
command (See section Optional Warnings and Messages).
The messages currently printed, and their meanings, are:
inner block not inside outer block in symbol
The symbol information shows where symbol scopes begin and end (such as at the start of a function or a block of statements). This error indicates that an inner scope block is not fully contained in its outer scope blocks.
_GDBN__ circumvents the problem by treating the inner block as if it had
the same scope as the outer block. In the error message, symbol
may be shown as “(don't know)
” if the outer block is not a
function.
block at address out of order
The symbol information for symbol scope blocks should occur in order of increasing addresses. This error indicates that it does not do so.
_GDBN__ does not circumvent this problem, and will have trouble locating
symbols in the source file whose symbols being read. (You can often
determine what source file is affected by specifying set verbose
on
. See section Optional Warnings and Messages.)
bad block start address patched
The symbol information for a symbol scope block has a start address smaller than the address of the preceding source line. This is known to occur in the SunOS 4.1.1 (and earlier) C compiler.
_GDBN__ circumvents the problem by treating the symbol scope block as starting on the previous source line.
bad string table offset in symbol n
Symbol number n contains a pointer into the string table which is larger than the size of the string table.
_GDBN__ circumvents the problem by considering the symbol to have the
name foo
, which may cause other problems if many symbols end up
with this name.
unknown symbol type 0xnn
The symbol information contains new data types that _GDBN__ does not yet
know how to read. 0xnn
is the symbol type of the misunderstood
information, in hexadecimal.
_GDBN__ circumvents the error by ignoring this symbol information. This
will usually allow the program to be debugged, though certain symbols
will not be accessible. If you encounter such a problem and feel like
debugging it, you can debug _GDBP__
with itself, breakpoint on
complain
, then go up to the function read_dbx_symtab
and
examine *bufp
to see the symbol.
stub type has NULL name
_GDBN__ could not find the full definition for a struct or class.
const/volatile indicator missing (ok if using g++ v1.x), got…
The symbol information for a C++ member function is missing some information that recent versions of the compiler should have output for it.
info mismatch between compiler and debugger
_GDBN__ could not parse a type specification output by the compiler.
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A target is the execution environment occupied by your program.
Often, _GDBN__ runs in the same host environment as the program you are
debugging; in that case, the debugging target is specified as a side
effect when you use the file
or core
commands. When you
need more flexibility—for example, running _GDBN__ on a physically
separate host, or controlling a standalone system over a serial port or
a realtime system over a TCP/IP connection—you can use the
target
command to specify one of the target types configured for
_GDBN__ (see section Commands for Managing Targets).
13.1 Active Targets | ||
13.2 Commands for Managing Targets | ||
13.3 Remote Debugging |
[ << ] | [ < ] | [ Up ] | [ > ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
There are three classes of targets: processes, core files, and executable files. _GDBN__ can work concurrently on up to three active targets, one in each class. This allows you to (for example) start a process and inspect its activity without abandoning your work on a core file.
If, for example, you execute ‘gdb a.out’, then the executable file
a.out
is the only active target. If you designate a core file as
well—presumably from a prior run that crashed and coredumped—then
_GDBN__ has two active targets and will use them in tandem, looking
first in the corefile target, then in the executable file, to satisfy
requests for memory addresses. (Typically, these two classes of target
are complementary, since core files contain only the program’s
read-write memory—variables and so on—plus machine status, while
executable files contain only the program text and initialized data.)
When you type run
, your executable file becomes an active process
target as well. When a process target is active, all _GDBN__ commands
requesting memory addresses refer to that target; addresses in an active
core file or executable file target are obscured while the process
target is active.
Use the core-file
, and exec-file
commands to select a new
core file or executable target (see section Commands to Specify Files). To specify as a target
a process that’s already running, use the attach
command
(see section Debugging an Already-Running Process).
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target type parameters
Connects the _GDBN__ host environment to a target machine or process. A target is typically a protocol for talking to debugging facilities. You use the argument type to specify the type or protocol of the target machine.
Further parameters are interpreted by the target protocol, but typically include things like device names or host names to connect with, process numbers, and baud rates.
The target
command will not repeat if you press <RET> again
after executing the command.
help target
Displays the names of all targets available. To display targets
currently selected, use either info target
or info files
(see section Commands to Specify Files).
help target name
Describe a particular target, including any parameters necessary to select it.
Here are some common targets (available, or not, depending on the _GDBN__ configuration):
target exec prog
An executable file. ‘target exec prog’ is the same as ‘exec-file prog’.
target core filename
A core dump file. ‘target core filename’ is the same as ‘core-file filename’.
target remote dev
Remote serial target in _GDBN__-specific protocol. The argument dev specifies what serial device to use for the connection (e.g. ‘/dev/ttya’). See section Remote Debugging.
_if__(_AMD29K__)
target amd-eb dev speed PROG
Remote PC-resident AMD EB29K board, attached over serial lines.
dev is the serial device, as for target remote
;
speed allows you to specify the linespeed; and PROG is the
name of the program to be debugged, as it appears to DOS on the PC.
@xref{EB29K Remote}.
_fi__(_AMD29K__) _if__(_I960__)
target nindy devicename
An Intel 960 board controlled by a Nindy Monitor. devicename is the name of the serial device to use for the connection, e.g. ‘/dev/ttya’. @xref{i960-Nindy Remote}.
_fi__(_I960__) _if__(_VXWORKS__)
target vxworks machinename
A VxWorks system, attached via TCP/IP. The argument machinename is the target system’s machine name or IP address. @xref{VxWorks Remote}. _fi__(_VXWORKS__)
_if__(_GENERIC__) Different targets are available on different configurations of _GDBN__; your configuration may have more or fewer targets. _fi__(_GENERIC__)
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_if__(_GENERIC__)
_include__(gdbinv-m.m4)<>_dnl__ |
---|
_fi__(_GENERIC__)
If you are trying to debug a program running on a machine that can’t run _GDBN__ in the usual way, it is often useful to use remote debugging. For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger.
Some configurations of _GDBN__ have special serial or TCP/IP interfaces to make this work with particular debugging targets. In addition, _GDBN__ comes with a generic serial protocol (specific to _GDBN__, but not specific to any particular target system) which you can use if you write the remote stubs—the code that will run on the remote system to communicate with _GDBN__.
To use the _GDBN__ remote serial protocol, the program to be debugged on the remote machine needs to contain a debugging stub which talks to _GDBN__ over the serial line. Several working remote stubs are distributed with _GDBN__; see the ‘README’ file in the _GDBN__ distribution for more information.
For details of this communication protocol, see the comments in the _GDBN__ source file ‘remote.c’.
To start remote debugging, first run _GDBN__ and specify as an executable file
the program that is running in the remote machine. This tells _GDBN__ how
to find the program’s symbols and the contents of its pure text. Then
establish communication using the target remote
command with a device
name as an argument. For example:
target remote /dev/ttyb
if the serial line is connected to the device named ‘/dev/ttyb’. This will stop the remote machine if it is not already stopped.
Now you can use all the usual commands to examine and change data and to step and continue the remote program.
To resume the remote program and stop debugging it, use the detach
command.
Other remote targets may be available in your
configuration of _GDBN__; use help targets
to list them.
_if__(_GENERIC__) _include__(gdbinv-s.m4) _fi__(_GENERIC__)
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You can alter many aspects of _GDBN__’s interaction with you by using
the set
command. For commands controlling how _GDBN__ displays
data, see section Print Settings; other settings are described here.
14.1 Prompt | ||
14.2 Command Editing | ||
14.3 Command History | ||
14.4 Screen Size | ||
14.5 Numbers | ||
14.6 Optional Warnings and Messages |
[ << ] | [ < ] | [ Up ] | [ > ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
_GDBN__ indicates its readiness to read a command by printing a string
called the prompt. This string is normally ‘(_GDBP__)’. You
can change the prompt string with the set prompt
command. For
instance, when debugging _GDBN__ with _GDBN__, it is useful to change
the prompt in one of the _GDBN__<>s so that you can always tell which
one you are talking to.
set prompt newprompt
Directs _GDBN__ to use newprompt as its prompt string henceforth.
show prompt
Prints a line of the form: ‘Gdb's prompt is: your-prompt’
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_GDBN__ reads its input commands via the readline interface. This
GNU library provides consistent behavior for programs which provide a
command line interface to the user. Advantages are emacs
-style
or vi
-style inline editing of commands, csh
-like history
substitution, and a storage and recall of command history across
debugging sessions.
You may control the behavior of command line editing in _GDBN__ with the
command set
.
set editing
set editing on
Enable command line editing (enabled by default).
set editing off
Disable command line editing.
show editing
Show whether command line editing is enabled.
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set history filename fname
Set the name of the _GDBN__ command history file to fname. This is
the file from which _GDBN__ will read an initial command history
list or to which it will write this list when it exits. This list is
accessed through history expansion or through the history
command editing characters listed below. This file defaults to the
value of the environment variable GDBHISTFILE
, or to
‘./.gdb_history’ if this variable is not set.
set history save
set history save on
Record command history in a file, whose name may be specified with the
set history filename
command. By default, this option is disabled.
set history save off
Stop recording command history in a file.
set history size size
Set the number of commands which _GDBN__ will keep in its history list.
This defaults to the value of the environment variable
HISTSIZE
, or to 256 if this variable is not set.
History expansion assigns special meaning to the character !.
Since ! is also the logical not operator in C, history expansion
is off by default. If you decide to enable history expansion with the
set history expansion on
command, you may sometimes need to
follow ! (when it is used as logical not, in an expression) with
a space or a tab to prevent it from being expanded. The readline
history facilities will not attempt substitution on the strings
!= and !(, even when history expansion is enabled.
The commands to control history expansion are:
set history expansion on
set history expansion
Enable history expansion. History expansion is off by default.
set history expansion off
Disable history expansion.
The readline code comes with more complete documentation of
editing and history expansion features. Users unfamiliar with emacs
or vi
may wish to read it.
show history
show history filename
show history save
show history size
show history expansion
These commands display the state of the _GDBN__ history parameters.
show history
by itself displays all four states.
show commands
Display the last ten commands in the command history.
show commands n
Print ten commands centered on command number n.
show commands +
Print ten commands just after the commands last printed.
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Certain commands to _GDBN__ may produce large amounts of information output to the screen. To help you read all of it, _GDBN__ pauses and asks you for input at the end of each page of output. Type <RET> when you want to continue the output. _GDBN__ also uses the screen width setting to determine when to wrap lines of output. Depending on what is being printed, it tries to break the line at a readable place, rather than simply letting it overflow onto the following line.
Normally _GDBN__ knows the size of the screen from the termcap data base
together with the value of the TERM
environment variable and the
stty rows
and stty cols
settings. If this is not correct,
you can override it with the set height
and set
width
commands:
set height lpp
show height
set width cpl
show width
These set
commands specify a screen height of lpp lines and
a screen width of cpl characters. The associated show
commands display the current settings.
If you specify a height of zero lines, _GDBN__ will not pause during output no matter how long the output is. This is useful if output is to a file or to an editor buffer.
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You can always enter numbers in octal, decimal, or hexadecimal in _GDBN__ by
the usual conventions: octal numbers begin with ‘0’, decimal
numbers end with ‘.’, and hexadecimal numbers begin with ‘0x’.
Numbers that begin with none of these are, by default, entered in base
10; likewise, the default display for numbers—when no particular
format is specified—is base 10. You can change the default base for
both input and output with the set radix
command.
set radix base
Set the default base for numeric input and display. Supported choices for base are decimal 2, 8, 10, 16. base must itself be specified either unambiguously or using the current default radix; for example, any of
set radix 1010 set radix 012 set radix 10. set radix 0xa
will set the base to decimal. On the other hand, ‘set radix 10’ will leave the radix unchanged no matter what it was.
show radix
Display the current default base for numeric input and display.
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By default, _GDBN__ is silent about its inner workings. If you are running
on a slow machine, you may want to use the set verbose
command.
It will make _GDBN__ tell you when it does a lengthy internal operation, so
you won’t think it has crashed.
Currently, the messages controlled by set verbose
are those which
announce that the symbol table for a source file is being read
(see section Commands to Specify Files, in the description of the command
symbol-file
).
set verbose on
Enables _GDBN__’s output of certain informational messages.
set verbose off
Disables _GDBN__’s output of certain informational messages.
show verbose
Displays whether set verbose
is on or off.
By default, if _GDBN__ encounters bugs in the symbol table of an object file, it is silent; but if you are debugging a compiler, you may find this information useful (see section Errors Reading Symbol Files).
set complaints limit
Permits _GDBN__ to output limit complaints about each type of unusual symbols before becoming silent about the problem. Set limit to zero to suppress all complaints; set it to a large number to prevent complaints from being suppressed.
show complaints
Displays how many symbol complaints _GDBN__ is permitted to produce.
By default, _GDBN__ is cautious, and asks what sometimes seem to be a lot of stupid questions to confirm certain commands. For example, if you try to run a program which is already running:
(_GDBP__) run The program being debugged has been started already. Start it from the beginning? (y or n)
If you’re willing to unflinchingly face the consequences of your own commands, you can disable this “feature”:
set confirm off
Disables confirmation requests.
set confirm on
Enables confirmation requests (the default).
show confirm
Displays state of confirmation requests.
Some systems allow individual object files that make up your program to be replaced without stopping and restarting your program. _if__(_VXWORKS__) For example, in VxWorks you can simply recompile a defective object file and keep on running. _fi__(_VXWORKS__) If you’re running on one of these systems, you can allow _GDBN__ to reload the symbols for automatically relinked modules:
set symbol-reloading on
Replace symbol definitions for the corresponding source file when an object file with a particular name is seen again.
set symbol-reloading off
Don’t replace symbol definitions when re-encountering object files of
the same name. This is the default state; if you’re not running on a
system that permits automatically relinking modules, you should leave
symbol-reloading
off, since otherwise _GDBN__ may discard symbols
when linking large programs, that may contain several modules (from
different directories or libraries) with the same name.
show symbol-reloading
Show the current on
or off
setting.
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Aside from breakpoint commands (see section Breakpoint Command Lists), _GDBN__ provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files.
15.1 User-Defined Commands | ||
15.2 Command Files | ||
15.3 Commands for Controlled Output |
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A user-defined command is a sequence of _GDBN__ commands to which you
assign a new name as a command. This is done with the define
command.
define commandname
Define a command named commandname. If there is already a command by that name, you are asked to confirm that you want to redefine it.
The definition of the command is made up of other _GDBN__ command lines,
which are given following the define
command. The end of these
commands is marked by a line containing end
.
document commandname
Give documentation to the user-defined command commandname. The
command commandname must already be defined. This command reads
lines of documentation just as define
reads the lines of the
command definition, ending with end
. After the document
command is finished, help
on command commandname will print
the documentation you have specified.
You may use the document
command again to change the
documentation of a command. Redefining the command with define
does not change the documentation.
help user-defined
List all user-defined commands, with the first line of the documentation (if any) for each.
info user
info user commandname
Display the _GDBN__ commands used to define commandname (but not its documentation). If no commandname is given, display the definitions for all user-defined commands.
User-defined commands do not take arguments. When they are executed, the commands of the definition are not printed. An error in any command stops execution of the user-defined command.
Commands that would ask for confirmation if used interactively proceed without asking when used inside a user-defined command. Many _GDBN__ commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.
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A command file for _GDBN__ is a file of lines that are _GDBN__ commands. Comments (lines starting with #) may also be included. An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal.
When you start _GDBN__, it automatically executes commands from its
init files. These are files named ‘_GDBINIT__’. _GDBN__
reads the init file (if any) in your home directory and then the init
file (if any) in the current working directory. (The init files are not
executed if you use the ‘-nx’ option; see section Choosing Modes.) You
can also request the execution of a command file with the source
command:
source filename
Execute the command file filename.
The lines in a command file are executed sequentially. They are not printed as they are executed. An error in any command terminates execution of the command file.
Commands that would ask for confirmation if used interactively proceed without asking when used in a command file. Many _GDBN__ commands that normally print messages to say what they are doing omit the messages when called from command files.
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During the execution of a command file or a user-defined command, normal _GDBN__ output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition. This section describes three commands useful for generating exactly the output you want.
echo text
Print text. Nonprinting characters can be included in text using C escape sequences, such as ‘\n’ to print a newline. No newline will be printed unless you specify one. In addition to the standard C escape sequences, a backslash followed by a space stands for a space. This is useful for outputting a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. Thus, to print ‘ and foo = ’, use the command ‘echo \ and foo = \ ’.
A backslash at the end of text can be used, as in C, to continue the command onto subsequent lines. For example,
echo This is some text\n\ which is continued\n\ onto several lines.\n
produces the same output as
echo This is some text\n echo which is continued\n echo onto several lines.\n
output expression
Print the value of expression and nothing but that value: no newlines, no ‘$nn = ’. The value is not entered in the value history either. See section Expressions for more information on expressions.
output/fmt expression
Print the value of expression in format fmt. You can use
the same formats as for print
; see section Output formats, for more
information.
printf string, expressions…
Print the values of the expressions under the control of string. The expressions are separated by commas and may be either numbers or pointers. Their values are printed as specified by string, exactly as if the program were to execute
printf (string, expressions…);
For example, you can print two values in hex like this:
printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo
The only backslash-escape sequences that you can use in the format string are the simple ones that consist of backslash followed by a letter.
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A special interface allows you to use GNU Emacs to view (and edit) the source files for the program you are debugging with _GDBN__.
To use this interface, use the command M-x gdb in Emacs. Give the executable file you want to debug as an argument. This command starts _GDBN__ as a subprocess of Emacs, with input and output through a newly created Emacs buffer.
Using _GDBN__ under Emacs is just like using _GDBN__ normally except for two things:
This applies both to _GDBN__ commands and their output, and to the input and output done by the program you are debugging.
This is useful because it means that you can copy the text of previous commands and input them again; you can even use parts of the output in this way.
All the facilities of Emacs’ Shell mode are available for interacting with your program. In particular, you can send signals the usual way—for example, C-c C-c for an interrupt, C-c C-z for a stop.
Each time _GDBN__ displays a stack frame, Emacs automatically finds the source file for that frame and puts an arrow (_0__‘=>’_1__) at the left margin of the current line. Emacs uses a separate buffer for source display, and splits the window to show both your _GDBN__ session and the source.
Explicit _GDBN__ list
or search commands still produce output as
usual, but you probably will have no reason to use them.
Warning: If the directory where your program resides is not your current directory, it can be easy to confuse Emacs about the location of the source files, in which case the auxiliary display buffer will not appear to show your source. _GDBN__ can find programs by searching your environment’s
PATH
variable, so the _GDBN__ input and output session will proceed normally; but Emacs doesn’t get enough information back from _GDBN__ to locate the source files in this situation. To avoid this problem, either start _GDBN__ mode from the directory where your program resides, or specify a full path name when prompted for the M-x gdb argument.A similar confusion can result if you use the _GDBN__
file
command to switch to debugging a program in some other location, from an existing _GDBN__ buffer in Emacs.
By default, M-x gdb calls the program called ‘gdb’. If
you need to call _GDBN__ by a different name (for example, if you keep
several configurations around, with different names) you can set the
Emacs variable gdb-command-name
; for example,
(setq gdb-command-name "mygdb")
(preceded by ESC ESC, or typed in the *scratch*
buffer, or
in your ‘.emacs’ file) will make Emacs call the program named
“mygdb
” instead.
In the _GDBN__ I/O buffer, you can use these special Emacs commands in addition to the standard Shell mode commands:
Describe the features of Emacs’ _GDBN__ Mode.
Execute to another source line, like the _GDBN__ step
command; also
update the display window to show the current file and location.
Execute to next source line in this function, skipping all function
calls, like the _GDBN__ next
command. Then update the display window
to show the current file and location.
Execute one instruction, like the _GDBN__ stepi
command; update
display window accordingly.
Execute to next instruction, using the _GDBN__ nexti
command; update
display window accordingly.
Execute until exit from the selected stack frame, like the _GDBN__
finish
command.
Continue execution of the program, like the _GDBN__ continue
command. Warning: In Emacs v19, this command is C-c C-p.
Go up the number of frames indicated by the numeric argument
(see Numeric Arguments in The GNU Emacs Manual),
like the _GDBN__ up
command. Warning: In Emacs v19, this
command is C-c C-u.
Go down the number of frames indicated by the numeric argument, like the
_GDBN__ down
command. Warning: In Emacs v19, this command
is C-c C-d.
Read the number where the cursor is positioned, and insert it at the end
of the _GDBN__ I/O buffer. For example, if you wish to disassemble code
around an address that was displayed earlier, type disassemble;
then move the cursor to the address display, and pick up the
argument for disassemble
by typing C-x &.
You can customize this further on the fly by defining elements of the list
gdb-print-command
; once it is defined, you can format or
otherwise process numbers picked up by C-x & before they are
inserted. A numeric argument to C-x & will both indicate that you
wish special formatting, and act as an index to pick an element of the
list. If the list element is a string, the number to be inserted is
formatted using the Emacs function format
; otherwise the number
is passed as an argument to the corresponding list element.
In any source file, the Emacs command C-x SPC (gdb-break
)
tells _GDBN__ to set a breakpoint on the source line point is on.
If you accidentally delete the source-display buffer, an easy way to get
it back is to type the command f
in the _GDBN__ buffer, to
request a frame display; when you run under Emacs, this will recreate
the source buffer if necessary to show you the context of the current
frame.
The source files displayed in Emacs are in ordinary Emacs buffers which are visiting the source files in the usual way. You can edit the files with these buffers if you wish; but keep in mind that _GDBN__ communicates with Emacs in terms of line numbers. If you add or delete lines from the text, the line numbers that _GDBN__ knows will cease to correspond properly to the code.
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Your bug reports play an essential role in making _GDBN__ reliable.
Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case the principal function of a bug report is to help the entire community by making the next version of _GDBN__ work better. Bug reports are your contribution to the maintenance of _GDBN__.
In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.
17.1 Have You Found a Bug? | ||
17.2 How to Report Bugs |
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If you are not sure whether you have found a bug, here are some guidelines:
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A number of companies and individuals offer support for GNU products. If you obtained _GDBN__ from a support organization, we recommend you contact that organization first.
Contact information for many support companies and individuals is available in the file ‘etc/SERVICE’ in the GNU Emacs distribution.
In any event, we also recommend that you send bug reports for _GDBN__ to one of these addresses:
bug-gdb@prep.ai.mit.edu {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gdb
Do not send bug reports to ‘info-gdb’, or to ‘help-gdb’, or to any newsgroups. Most users of _GDBN__ do not want to receive bug reports. Those that do, have arranged to receive ‘bug-gdb’.
The mailing list ‘bug-gdb’ has a newsgroup ‘gnu.gdb.bug’ which serves as a repeater. The mailing list and the newsgroup carry exactly the same messages. Often people think of posting bug reports to the newsgroup instead of mailing them. This appears to work, but it has one problem which can be crucial: a newsgroup posting often lacks a mail path back to the sender. Thus, if we need to ask for more information, we may be unable to reach you. For this reason, it is better to send bug reports to the mailing list.
As a last resort, send bug reports on paper to:
GNU Debugger Bugs Free Software Foundation 545 Tech Square Cambridge, MA 02139
The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and assume that some details don’t matter. Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it doesn’t, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the debugger into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us. It isn’t as important what happens if the bug is already known. Therefore, always write your bug reports on the assumption that the bug has not been reported previously.
Sometimes people give a few sketchy facts and ask, “Does this ring a bell?” Those bug reports are useless, and we urge everyone to refuse to respond to them except to chide the sender to report bugs properly.
To enable us to fix the bug, you should include all these things:
show version
.
Without this, we won’t know whether there is any point in looking for the bug in the current version of _GDBN__.
If we were to try to guess the arguments, we would probably guess wrong and then we might not encounter the bug.
Of course, if the bug is that _GDBN__ gets a fatal signal, then we will certainly notice it. But if the bug is incorrect output, we might not notice unless it is glaringly wrong. We are human, after all. You might as well not give us a chance to make a mistake.
Even if the problem you experience is a fatal signal, you should still say so explicitly. Suppose something strange is going on, such as, your copy of _GDBN__ is out of synch, or you have encountered a bug in the C library on your system. (This has happened!) Your copy might crash and ours would not. If you told us to expect a crash, then when ours fails to crash, we would know that the bug was not happening for us. If you had not told us to expect a crash, then we would not be able to draw any conclusion from our observations.
The line numbers in our development sources won’t match those in your sources. Your line numbers would convey no useful information to us.
Here are some things that are not necessary:
Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.
This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. We recommend that you save your time for something else.
Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us. Errors in the output will be easier to spot, running under the debugger will take less time, etc.
However, simplification is not vital; if you don’t want to do this, report the bug anyway and send us the entire test case you used.
A patch for the bug does help us if it is a good one. But don’t omit the necessary information, such as the test case, on the assumption that a patch is all we need. We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as _GDBN__ it is very hard to construct an example that will make the program follow a certain path through the code. If you don’t send us the example, we won’t be able to construct one, so we won’t be able to verify that the bug is fixed.
And if we can’t understand what bug you are trying to fix, or why your patch should be an improvement, we won’t install it. A test case will help us to understand.
Such guesses are usually wrong. Even we can’t guess right about such things without first using the debugger to find the facts.
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The following commands were renamed in _GDBN__ 4.0, in order to make the command set as a whole more consistent and easier to use and remember:
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_GDBN__ comes with a configure
script that automates the process
of preparing _GDBN__ for installation; you can then use make
to
build the _GDBP__
program.
The _GDBP__ distribution includes all the source code you need for _GDBP__ in a single directory ‘gdb-_GDB_VN__’. That directory in turn contains:
gdb-_GDB_VN__/configure (and supporting files)
script for configuring _GDBN__ and all its supporting libraries.
gdb-_GDB_VN__/gdb
the source specific to _GDBN__ itself
gdb-_GDB_VN__/bfd
source for the Binary File Descriptor Library
gdb-_GDB_VN__/include
GNU include files
gdb-_GDB_VN__/libiberty
source for the ‘-liberty’ free software library
gdb-_GDB_VN__/readline
source for the GNU command-line interface
It is most convenient to run configure
from the ‘gdb-_GDB_VN__’
directory. The simplest way to configure and build _GDBN__ is the
following:
cd gdb-_GDB_VN__ ./configure host make
where host is something like ‘sun4’ or ‘decstation’, that
identifies the platform where _GDBN__ will run. This builds the three
libraries ‘bfd’, ‘readline’, and ‘libiberty’, then
gdb
itself. The configured source files, and the binaries, are
left in the corresponding source directories.
configure
is a Bourne-shell (/bin/sh
) script; if your
system doesn’t recognize this automatically when you run a different
shell, you may need to run sh
on it explicitly:
‘sh configure host’.
You can run the configure
script from any of the
subordinate directories in the _GDBN__ distribution (if you only want to
configure that subdirectory); but be sure to specify a path to it. For
example, to configure only the bfd
subdirectory,
cd gdb-_GDB_VN__/bfd ../configure host
You can install _GDBP__
anywhere; it has no hardwired paths. However,
you should make sure that the shell on your path (named by the
‘SHELL’ environment variable) is publicly readable; some systems
refuse to let _GDBN__ debug child processes whose programs are not
readable, and _GDBN__ uses the shell to start your program.
B.1 Configuration Subdirectories | Configuration subdirectories | |
B.2 Specifying Names for Hosts and Targets | Specifying names for hosts and targets | |
B.3 configure Options | Summary of options for configure | |
B.4 Formatting the Documentation | How to format and print _GDBN__ documentation |
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If you want to run _GDBN__ versions for several host or target machines,
you’ll need a different _GDBP__ compiled for each combination of host
and target. configure
is designed to make this easy by allowing
you to generate each configuration in a separate subdirectory. If your
make
program handles the ‘VPATH’ feature (GNU make
does), running make
in each of these directories then builds the
_GDBP__ program specified there.
configure
creates these subdirectories for you when you
simultaneously specify several configurations; but it’s a good habit
even for a single configuration. You can specify the use of
subdirectories using the ‘+subdirs’ option (abbreviated
‘+sub’). For example, you can build _GDBN__ this way on a Sun 4 as
follows:
cd gdb-_GDB_VN__ ./configure +sub sun4 cd H-sun4/T-sun4 make
When configure
uses subdirectories to build programs or
libraries, it creates nested directories
‘H-host/T-target’. configure
uses these two
directory levels because _GDBN__ can be configured for cross-compiling:
_GDBN__ can run on one machine (the host) while debugging programs that
run on another machine (the target). You specify cross-debugging
targets by giving the ‘+target=target’ option to
configure
. Specifying only hosts still gives you two levels of
subdirectory for each host, with the same configuration suffix on both;
that is, if you give any number of hosts but no targets, _GDBN__ will be
configured for native debugging on each host. On the other hand,
whenever you specify both hosts and targets on the same command line,
configure
creates all combinations of the hosts and targets you
list.
If you run configure
from a directory (notably,
‘gdb-_GDB_VN__’) that contains source directories for multiple
libraries or programs, configure
creates the
‘H-host/T-target’ subdirectories in each library or
program’s source directory. For example, typing:
cd gdb-_GDB_VN__ configure sun4 +target=vxworks960
creates the following directories:
gdb-_GDB_VN__/H-sun4/T-vxworks960 gdb-_GDB_VN__/bfd/H-sun4/T-vxworks960 gdb-_GDB_VN__/gdb/H-sun4/T-vxworks960 gdb-_GDB_VN__/libiberty/H-sun4/T-vxworks960 gdb-_GDB_VN__/readline/H-sun4/T-vxworks960
When you run make
to build a program or library, you must run it
in a configured directory. If you made a single configuration,
without subdirectories, run make
in the source directory.
If you have ‘H-host/T-target’ subdirectories,
run make
in those subdirectories.
The Makefile
generated by configure
for each source
directory runs recursively, so that typing make
in
‘gdb-_GDB_VN__’ (or in a
‘gdb-_GDB_VN__/H-host/T-target’ subdirectory) builds
all the required libraries, then _GDBN__.
When you have multiple hosts or targets configured, you can run
make
on them in parallel (for example, if they are NFS-mounted on
each of the hosts); they will not interfere with each other.
You can also use the ‘+objdir=altroot’ option to have the
configured files placed in a parallel directory structure rather than
alongside the source files; see section configure
Options.
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The specifications used for hosts and targets in the configure
script are based on a three-part naming scheme, but some short predefined
aliases are also supported. The full naming scheme encodes three pieces
of information in the following pattern:
architecture-vendor-os
For example, you can use the alias sun4
as a host argument
or in a +target=target
option, but the equivalent full name
is ‘sparc-sun-sunos4’.
The following table shows all the architectures, hosts, and OS prefixes
that configure
recognizes in _GDBN__ _GDB_VN__. Entries in the “OS
prefix” column ending in a ‘*’ may be followed by a release number.
The configure
script accompanying _GDBN__ _GDB_VN__ does not provide
any query facility to list all supported host and target names or
aliases. configure
calls the Bourne shell script
config.sub
to map abbreviations to full names; you can read the
script, if you wish, or you can use it to test your guesses on
abbreviations—for example:
% sh config.sub sun4 sparc-sun-sunos4 % sh config.sub sun3 m68k-sun-sunos4 % sh config.sub decstation mips-dec-ultrix % sh config.sub hp300bsd m68k-hp-bsd % sh config.sub i386v i386-none-sysv % sh config.sub i486v *** Configuration "i486v" not recognized
config.sub
is also distributed in the directory ‘gdb-_GDB_VN__’.
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configure
OptionsHere is a summary of all the configure
options and arguments that
you might use for building _GDBN__:
configure [+destdir=dir] [+subdirs] [+objdir=altroot] [+norecursion] [+rm] [+target=target…] host…
You may introduce options with the character ‘-’ rather than ‘+’ if you prefer; but you may abbreviate option names if you use ‘+’.
+destdir=dir
dir is an installation directory path prefix. After you
configure with this option, make install
will install _GDBN__ as
‘dir/bin/_GDBP__’, and the libraries in ‘dir/lib’.
If you specify ‘+destdir=/usr/local’, for example, make
install
creates ‘/usr/local/bin/gdb’.
+subdirs
Write configuration specific files in subdirectories of the form
H-host/T-target
(and configure the Makefile
to generate object code in
subdirectories of this form as well). Without this option, if you
specify only one configuration for _GDBN__, configure
will use
the same directory for source, configured files, and binaries. This
option is used automatically if you specify more than one host or
more than one ‘+target=target’ option on the configure
command line.
+norecursion
Configure only the directory where configure
is executed; do not
propagate configuration to subdirectories.
+objdir=altroot
altroot is an alternative directory used as the root for
configured files. configure
will create directories under
altroot in parallel to the source directories. If you use
‘+objdir=altroot’ with ‘+subdirs’, configure
also
builds the ‘H-host/T-target’ subdirectories in the
directory tree rooted in altroot.
+rm
Remove the configuration that the other arguments specify.
+target=target …
Configure _GDBN__ for cross-debugging programs running on each specified target. You may specify as many ‘+target’ options as you wish. Without this option, _GDBN__ is configured to debug programs that run on the same machine (host) as _GDBN__ itself.
There is no convenient way to generate a list of all available targets.
host …
Configure _GDBN__ to run on each specified host. You may specify as many host names as you wish.
There is no convenient way to generate a list of all available hosts.
configure
accepts other options, for compatibility with
configuring other GNU tools recursively; but these are the only
options that affect _GDBN__ or its supporting libraries.
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The _GDBN__ _GDB_VN__ release includes an already-formatted reference card, ready for printing on a PostScript printer, as ‘gdb-_GDB_VN__/gdb/refcard.ps’. It uses the most common PostScript fonts: the Times family, Courier, and Symbol. If you have a PostScript printer, you can print the reference card by just sending ‘refcard.ps’ to the printer.
The release also includes the online Info version of this manual already formatted: the main Info file is ‘gdb-_GDB_VN__/gdb/gdb.info’, and it refers to subordinate files matching ‘gdb.info*’ in the same directory.
If you want to make these Info files yourself from the _GDBN__ manual’s
source, you need the GNU makeinfo
program. Once you have it, you
can type
cd gdb-_GDB_VN__/gdb make gdb.info
to make the Info file.
If you want to format and print copies of the manual, you need several things:
dvips
, which can print
DVI files on PostScript printers.
Once you have these things, you can type
cd gdb-_GDB_VN__/gdb make gdb.dvi
to format the text of this manual, and print it with the usual output method for TeX DVI files at your site.
If you want to print the reference card, but don’t have a PostScript printer, or you want to use Computer Modern fonts instead, you can still print it if you have TeX. Format the reference card by typing
cd gdb-_GDB_VN__/gdb make refcard.dvi
The _GDBN__ reference card is designed to print in landscape mode on US “letter” size paper; that is, on a sheet 11 inches wide by 8.5 inches high. You will need to specify this form of printing as an option to your DVI output program.
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Version 2, June 1991
Copyright © 1989, 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
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The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software—to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation’s software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author’s protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors’ reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone’s free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
@alphaenumerate
These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.
In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.
@alphaenumerate
The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.
If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.
If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances.
It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and “any later version”, you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
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If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) 19yy name of author This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) 19yy name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.
The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than ‘show w’ and ‘show c’; they could even be mouse-clicks or menu items—whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a “copyright disclaimer” for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. signature of Ty Coon, 1 April 1989 Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
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This is a way of removing one
word from the stack, on machines where stacks grow downward in memory
(most machines, nowadays). This assumes that the innermost stack frame
is selected; setting $sp
is not allowed when other stack frames
are selected. To pop entire frames off the stack, regardless of
machine architecture, use return
; see section Returning from a Function.
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