{Using _GDBN__

{A Guide to the GNU Source-Level Debugger

_if__(!_GENERIC__)

{On _HOST__ Systems

_GDBN__ version _GDB_VN__

July 1991

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__)

 — The Detailed Node Listing —

Summary of _GDBN__

Getting In and Out of _GDBN__

Starting _GDBN__

_GDBN__ Commands

Running Programs Under _GDBN__

Stopping and Continuing

Breakpoints, Watchpoints, and Exceptions

Examining the Stack

Examining Source Files

Examining Data

Using GDB with Different Languages

Switching between source languages

Type and range Checking

Supported Languages

C and C++

Modula-2

Altering Execution

_GDBN__’s Files

Specifying a Debugging Target

Remote Debugging

_GDBN__ with a Remote i960 (Nindy)

_GDBN__ with a Remote EB29K

_GDBN__ and VxWorks

Controlling _GDBN__

Canned Sequences of Commands

Reporting Bugs in _GDBN__

Installing GDB

Summary of _GDBN__

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:

• Start your program, specifying anything that might affect its behavior.
• Make your program stop on specified conditions.
• Examine what has happened, when your program has stopped.
• Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.

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

_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.

Contributors to GDB

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.

New Features since _GDBN__ version 3.5

Targets

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.

Watchpoints

_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.

Wide Output

Commands that issue wide output now insert newlines at places designed to make the output more readable.

Object Code Formats

_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.

Configuration and Ports

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.

Interaction

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.

C++

_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.

Modula-2

_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.

Command Rationalization

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.

Shared Libraries

_GDBN__ _GDB_VN__ can debug programs and core files that use SunOS shared libraries.

Reference Card

_GDBN__ _GDB_VN__ has a reference card; See section Formatting the Documentation for instructions on printing it.

Work in Progress

Kernel debugging for BSD and Mach systems; Tahoe and HPPA architecture support.

1 A Sample _GDBN__ Session

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__

2 Getting In and Out of _GDBN__

2.1 Starting _GDBN__

_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

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__)

_fi__(!_GENERIC__) _if__(_GENERIC__) _fi__(_GENERIC__)

2.1.2 Choosing Modes

-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__)

2.2 Leaving _GDBN__

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.

2.3 Shell Commands

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’.

3 _GDBN__ Commands

3.1 Command Syntax

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).

3.2 Getting Help

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.

4 Running Programs Under _GDBN__

4.1 Compiling for Debugging

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.

4.2 Starting your Program

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:

The arguments.

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.

The environment.

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.

The working directory.

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.

The standard input and output.

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.

4.3 Your Program’s Arguments

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.

4.4 Your Program’s Environment

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.

4.5 Your Program’s Working Directory

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.

4.6 Your Program’s Input and Output

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.

4.7 Debugging an Already-Running Process

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).

4.8 Killing the Child Process

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).

5 Stopping and Continuing

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

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.

5.1.1 Setting Breakpoints

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).

5.1.2 Setting Watchpoints

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.

5.1.3 Breakpoints and Exceptions

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.

• If you call a function interactively, _GDBN__ normally returns control to you when the function has finished executing. If the call raises an exception, however, the call may bypass the mechanism that returns control to the user and cause the program to simply continue running until it hits a breakpoint, catches a signal that _GDBN__ is listening for, or exits.
• You cannot raise an exception interactively.
• You cannot interactively install an exception handler.

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.

5.1.4 Deleting Breakpoints

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.

5.1.5 Disabling Breakpoints

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:

• Enabled. The breakpoint will stop the program. A breakpoint set with the break command starts out in this state.
• Disabled. The breakpoint has no effect on the program.
• Enabled once. The breakpoint will stop the program, but when it does so it will become disabled. A breakpoint set with the tbreak command starts out in this state.
• Enabled for deletion. The breakpoint will stop the program, but immediately after it does so it will be deleted permanently.

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.)

5.1.6 Break Conditions

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.

5.1.7 Breakpoint Command Lists

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.

5.1.8 Breakpoint Menus

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__)

5.1.9 “Cannot Insert Breakpoints”

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:

1. Remove or disable the breakpoints, then continue.
2. Suspend _GDBN__, and copy the file containing the program to a new name. Resume _GDBN__ and use the exec-file command to specify that _GDBN__ should run the program under that name. Then start the program again.
3. Relink the program so that the text segment is nonsharable, using the linker option ‘-N’. The operating system limitation may not apply to nonsharable executables.

5.2 Continuing and Stepping

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.

5.3 Signals

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.

6 Examining the Stack

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

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.

6.2 Backtraces

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.

6.3 Selecting a Frame

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.

6.4 Information About a Frame

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.

7 Examining Source Files

_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

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.

7.2 Searching Source Files

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.

7.3 Specifying Source Directories

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:

1. Use directory with no argument to reset the source path to empty.
2. Use directory with suitable arguments to reinstall the directories you want in the source path. You can add all the directories in one command.

7.4 Source and Machine Code

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__)

8 Examining Data

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.

8.1 Expressions

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.

8.2 Program Variables

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.

8.3 Artificial Arrays

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>
…

8.4 Output formats

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.

8.5 Examining Memory

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.

8.6 Automatic Display

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.

8.7 Print Settings

_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.

8.8 Value History

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 +’.

8.9 Convenience Variables

_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.

8.10 Registers

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.

8.11 Floating Point Hardware

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.

9 Using _GDBN__ with Different Languages

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

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

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.

9.1.2 Having _GDBN__ infer the source language

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:

‘*.mod’

Modula-2 source file

‘*.c’

‘*.cc’

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.

9.2 Displaying the language

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.

9.3 Type and range Checking

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

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.

9.3.2 An overview of Range Checking

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__.

9.4 Supported Languages

_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++

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

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:

• Integral types include int with any of its storage-class specifiers, char, and enums.
• Floating-point types include float and double.
• Pointer types include all types defined as (type *).
• Scalar types include all of the above.

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 structs and unions.

[]

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__

9.4.1.2 C and C++ Constants

_GDBN__ allows you to express the constants of C and C++ in the following ways:

• Integer constants are a sequence of digits. Octal constants are specified by a leading ‘0’ (ie. zero), and hexadecimal constants by a leading ‘0x’ or ‘0X’. Constants may also end with an ‘l’, specifying that the constant should be treated as a long value.
• Floating point constants are a sequence of digits, followed by a decimal point, followed by a sequence of digits, and optionally followed by an exponent. An exponent is of the form: ‘e[[+]|-]nnn’, where nnn is another sequence of digits. The ‘+’ is optional for positive exponents.
• Enumerated constants consist of enumerated identifiers, or their integral equivalents.
• Character constants are a single character surrounded by single quotes ('), 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.
• String constants are a sequence of character constants surrounded by double quotes (").
• Pointer constants are an integral value.

9.4.1.3 C++ Expressions

_GDBN__’s expression handling has the following extensions to interpret a significant subset of C++ expressions:

2. Member function calls are allowed; you can use expressions like

   count = aml->GetOriginal(x, y)
   

3. While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, _GDBN__ allows implicit references to the class instance pointer this following the same rules as C++.
4. You can call overloaded functions; _GDBN__ will resolve the function call to the right definition, with one restriction—you must use arguments of the type required by the function that you want to call. _GDBN__ will not perform conversions requiring constructors or user-defined type operators.
5. _GDBN__ understands variables declared as C++ references; you can use them in expressions just as you do in C++ source—they are automatically dereferenced.

   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’.

6. _GDBN__ supports the C++ name resolution operator ::—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.

9.4.1.4 C and C++ Defaults

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.

9.4.1.5 C and C++ Type and Range Checks

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:

• The two variables are structured and have the same structure, union, or enumerated tag.
• Two two variables have the same type name, or types that have been declared equivalent through 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.

9.4.1.6 _GDBN__ and C

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)

9.4.1.7 _GDBN__ Commands for C++

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.

9.4.2 Modula-2

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

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:

• Integral types consist of INTEGER, CARDINAL, and their subranges.
• Character types consist of CHAR and its subranges.
• Floating-point types consist of REAL.
• Pointer types consist of anything declared as POINTER TO type.
• Scalar types consist of all of the above.
• Set types consist of SETs and BITSETs.
• Boolean types consist of 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 INTEGERs and REALs.

^

Pointer dereferencing. Defined on pointer types.

NOT

Boolean negation. Defined on boolean types. Same precedence as ^.

.

RECORD field selector. Defined on RECORDs. Same precedence as ^.

[]

Array indexing. Defined on ARRAYs. Same precedence as ^.

()

Procedure argument list. Defined on PROCEDUREs. 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__

9.4.2.2 Built-in Functions and Procedures

Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:

a

represents an ARRAY variable.

c

represents a CHAR constant or variable.

i

represents a variable or constant of integral type.

m

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.

n

represents a variable or constant of integral or floating-point type.

r

represents a variable or constant of floating-point type.

t

represents a type.

v

represents a variable.

x

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 and EXCL as an error.

9.4.2.3 Constants

_GDBN__ allows you to express the constants of Modula-2 in the following ways:

• Integer constants are simply a sequence of digits. When used in an expression, a constant is interpreted to be type-compatible with the rest of the expression. Hexadecimal integers are specified by a trailing ‘H’, and octal integers by a trailing ‘B’.
• Floating point constants appear as a sequence of digits, followed by a decimal point and another sequence of digits. An optional exponent can then be specified, in the form ‘E[+|-]nnn’, where ‘[+|-]nnn’ is the desired exponent. All of the digits of the floating point constant must be valid decimal (base 10) digits.
• Character constants consist of a single character enclosed by a pair of like quotes, either single (') or double ("). They may also be expressed by their ordinal value (their ASCII value, usually) followed by a ‘C’.
• String constants consist of a sequence of characters enclosed by a pair of like quotes, either single (') 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.
• Enumerated constants consist of an enumerated identifier.
• Boolean constants consist of the identifiers TRUE and FALSE.
• Pointer constants consist of integral values only.
• Set constants are not yet supported.

9.4.2.4 Modula-2 Defaults

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.

9.4.2.5 Deviations from Standard Modula-2

A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:

• Unlike in standard Modula-2, pointer constants can be formed by integers. This allows you to modify pointer variables during debugging. (In standard Modula-2, the actual address contained in a pointer variable is hidden from you; it can only be modified through direct assignment to another pointer variable or expression that returned a pointer.)
• C escape sequences can be used in strings and characters to represent non-printable characters. _GDBN__ will print out strings with these escape sequences embedded. Single non-printable characters are printed using the ‘CHR(nnn)’ format.
• The assignment operator (:=) returns the value of its right-hand argument.
• All builtin procedures both modify and return their argument.

9.4.2.6 Modula-2 Type and Range Checks

Warning: in this release, _GDBN__ does not yet perform type or range checking.

_GDBN__ considers two Modula-2 variables type equivalent if:

• They are of types that have been declared equivalent via a TYPE t1 = t2 statement
• They have been declared on the same line. (Note: This is true of the GNU Modula-2 compiler, but it may not be true of other compilers.)

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.

9.4.2.7 The scope operators :: 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.

9.4.2.8 _GDBN__ and Modula-2

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__

10 Examining the Symbol Table

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.

11 Altering Execution

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

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.

11.2 Continuing at a Different Address

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.

11.3 Giving the Program a Signal

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.

11.4 Returning from a Function

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.

11.5 Calling your Program’s Functions

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.

11.6 Patching your Program

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.

12 _GDBN__’s Files

12.1 Commands to Specify Files

_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.