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This is Info file ./gdb.info, produced by Makeinfo-1.52 from the input file gdb.texinfo. START-INFO-DIR-ENTRY * Gdb: (gdb). The GNU debugger. END-INFO-DIR-ENTRY This file documents the GNU debugger GDB. This is Edition 4.09, April 1993, of `Debugging with GDB: the GNU Source-Level Debugger' for GDB Version 4.9. Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993 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. File: gdb.info, Node: Altering, Next: GDB Files, Prev: Symbols, Up: Top Altering Execution ****************** Once you think you have found an error in your 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 GDB features for altering execution of the program. For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function to its caller. * Menu: * Assignment:: Assignment to variables * Jumping:: Continuing at a different address * Signaling:: Giving your program a signal * Returning:: Returning from a function * Calling:: Calling your program's functions * Patching:: Patching your program File: gdb.info, Node: Assignment, Next: Jumping, Up: Altering Assignment to variables ======================= To alter the value of a variable, evaluate an assignment expression. *Note Expressions: Expressions. For example, print x=4 stores the value 4 into the variable `x', and then prints the value of the assignment expression (which is 4). *Note Using GDB with Different Languages: 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 (*note Value history: 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, if your program has a variable `width', you get an error if you try to set a new value with just `set width=13', because GDB has the command `set width': (gdb) whatis width type = double (gdb) p width $4 = 13 (gdb) set width=47 Invalid syntax in expression. The invalid expression, of course, is `=47'. In order to actually set the program's variable `width', use (gdb) set var width=47 GDB allows more implicit conversions in assignments than C; you can freely store an integer value into a pointer variable or vice versa, and you can convert any structure 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 (*note Expressions: 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. File: gdb.info, Node: Jumping, Next: Signaling, Prev: Assignment, Up: Altering Continuing at a different address ================================= Ordinarily, when you continue your 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. *Note Printing source lines: List, 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 your 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 your 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 your program stopped. *Note Continuing and stepping: 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. File: gdb.info, Node: Signaling, Next: Returning, Prev: Jumping, Up: Altering Giving your program a signal ============================ `signal SIGNALNUM' Resume execution where your 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 your 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. File: gdb.info, Node: Returning, Next: Calling, Prev: Signaling, Up: Altering 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', GDB 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 (*note Selecting a frame: Selection.), 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 (*note Continuing and stepping: Continuing and Stepping.) resumes execution until the selected stack frame returns naturally. File: gdb.info, Node: Calling, Next: Patching, Prev: Returning, Up: Altering Calling program 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. File: gdb.info, Node: Patching, Prev: Calling, Up: Altering Patching programs ================= By default, GDB 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', GDB will open executable and core files for both reading and writing; if you specify `set write off' (the default), GDB will open them read-only. If you have 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. File: gdb.info, Node: GDB Files, Next: Targets, Prev: Altering, Up: Top GDB Files ********* GDB 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 your program. To debug a core dump of a previous run, you must also tell GDB the name of the core dump file. * Menu: * Files:: Commands to specify files * Symbol Errors:: Errors reading symbol files File: gdb.info, Node: Files, Next: Symbol Errors, Up: GDB Files Commands to specify files ========================= The usual way to specify executable and core dump file names is with the command arguments given when you start GDB (*note Getting In and Out of GDB: Invocation.. Occasionally it is necessary to change to a different file during a GDB session. Or you may run GDB and forget to specify a file you want to use. In these situations the GDB 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 the GDB working directory, GDB 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 GDB and your program, using the `path' command. On systems with memory-mapped files, an auxiliary symbol table file `FILENAME.syms' may be available for FILENAME. If it is, GDB will map in the symbol table from `FILENAME.syms', starting up more quickly. See the descriptions of the options `-mapped' and `-readnow' (available on the command line, and with the commands `file', `symbol-file', or `add-symbol-file'), for more information. `file' `file' with no argument makes GDB 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. GDB will search the environment variable `PATH' if necessary to locate your 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 GDB information on your program's symbol table. The `symbol-file' command causes GDB 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 GDB. `symbol-file' will not repeat if you press RET again after executing it once. When GDB 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. On some kinds of object files, the `symbol-file' command does not normally 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, as they are needed. The purpose of this two-stage reading strategy is to make GDB 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. *Note Optional warnings and messages: Messages/Warnings.) We have not implemented the two-stage strategy for COFF yet. When the symbol table is stored in COFF format, `symbol-file' reads the symbol table data in full right away. `symbol-file FILENAME [ -readnow ] [ -mapped ]' `file FILENAME [ -readnow ] [ -mapped ]' You can override the GDB two-stage strategy for reading symbol tables by using the `-readnow' option with any of the commands that load symbol table information, if you want to be sure GDB has the entire symbol table available. If memory-mapped files are available on your system through the `mmap' system call, you can use another option, `-mapped', to cause GDB to write the symbols for your program into a reusable file. Future GDB debugging sessions will map in symbol information from this auxiliary symbol file (if the program has not changed), rather than spending time reading the symbol table from the executable program. Using the `-mapped' option has the same effect as starting GDB with the `-mapped' command-line option. You can use both options together, to make sure the auxiliary symbol file has all the symbol information for your program. The auxiliary symbol file for a program called MYPROG is called `MYPROG.syms'. Once this file exists (so long as it is newer than the corresponding executable), GDB will always attempt to use it when you debug MYPROG; no special options or commands are needed. The `.syms' file is specific to the host machine where you run GDB. It holds an exact image of the internal GDB symbol table. It cannot be shared across multiple host platforms. `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; GDB 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 GDB. So, if you have been running your 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 (*note Killing the child process: Kill Process.). `load FILENAME' Depending on what remote debugging facilities are configured into GDB, 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 the FILENAME symbol table in GDB, like the `add-symbol-file' command. If your GDB does not have a `load' command, attempting to execute it gets the error message "`You can't do that when your target is ...'" On VxWorks, `load' will dynamically link FILENAME on the current target system as well as adding its symbols in GDB. With the Nindy interface to an Intel 960 board, `load' will download FILENAME to the 960 as well as adding its symbols in GDB. When you select remote debugging to a Hitachi SH, H8/300, or H8/500 board (*note GDB and Hitachi Microprocessors: Hitachi Remote.), the `load' command downloads your program to the Hitachi board and also opens it as the current executable target for GDB on your host (like the `file' command). `load' will not repeat if you press RET again after using it. `add-symbol-file FILENAME ADDRESS' `add-symbol-file FILENAME ADDRESS [ -readnow ] [ -mapped ]' 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; GDB 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. You can use the `-mapped' and `-readnow' options just as with the `symbol-file' command, to change how GDB manages the symbol table information for FILENAME. `info files' `info target' `info files' and `info target' are synonymous; both print the current target (*note Specifying a Debugging Target: Targets.), including the names of the executable and core dump files currently in use by GDB, 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. GDB always converts the file name to an absolute path name and remembers it that way. GDB supports SunOS, SVR4, and IBM RS/6000 shared libraries. GDB 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, GDB will not understand references to a function in a shared library, however--unless you are 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. File: gdb.info, Node: Symbol Errors, Prev: Files, Up: GDB Files Errors reading symbol files =========================== While reading a symbol file, GDB will occasionally encounter problems, such as symbol types it does not recognize, or known bugs in compiler output. By default, GDB does not notify you of such problems, since they are relatively common and primarily of interest to people debugging compilers. If you are interested in seeing information about ill-constructed symbol tables, you can either ask GDB to print only one message about each such type of problem, no matter how many times the problem occurs; or you can ask GDB to print more messages, to see how many times the problems occur, with the `set complaints' command (*note Optional warnings and messages: Messages/Warnings.). The messages currently printed, and their meanings, are: `inner block not inside outer block in SYMBOL' The symbol information shows where symbol scopes begin and end (such as at the start of a function or a block of statements). This error indicates that an inner scope block is not fully contained in its outer scope blocks. GDB circumvents the problem by treating the inner block as if it had the same scope as the outer block. In the error message, SYMBOL may be shown as "`(don't know)'" if the outer block is not a function. `block at ADDRESS out of order' The symbol information for symbol scope blocks should occur in order of increasing addresses. This error indicates that it does not do so. GDB does not circumvent this problem, and will have trouble locating symbols in the source file whose symbols it is reading. (You can often determine what source file is affected by specifying `set verbose on'. *Note Optional warnings and messages: Messages/Warnings.) `bad block start address patched' The symbol information for a symbol scope block has a start address smaller than the address of the preceding source line. This is known to occur in the SunOS 4.1.1 (and earlier) C compiler. GDB circumvents the problem by treating the symbol scope block as starting on the previous source line. `bad string table offset in symbol N' Symbol number N contains a pointer into the string table which is larger than the size of the string table. GDB circumvents the problem by considering the symbol to have the name `foo', which may cause other problems if many symbols end up with this name. `unknown symbol type `0xNN'' The symbol information contains new data types that GDB does not yet know how to read. `0xNN' is the symbol type of the misunderstood information, in hexadecimal. GDB circumvents the error by ignoring this symbol information. This will usually allow your program to be debugged, though certain symbols will not be accessible. If you encounter such a problem and feel like debugging it, you can debug `gdb' with itself, breakpoint on `complain', then go up to the function `read_dbx_symtab' and examine `*bufp' to see the symbol. `stub type has NULL name' GDB could not find the full definition for a struct or class. `const/volatile indicator missing (ok if using g++ v1.x), got...' The symbol information for a C++ member function is missing some information that recent versions of the compiler should have output for it. `info mismatch between compiler and debugger' GDB could not parse a type specification output by the compiler. File: gdb.info, Node: Targets, Next: Controlling GDB, Prev: GDB Files, Up: Top Specifying a Debugging Target ***************************** A "target" is the execution environment occupied by your program. Often, GDB runs in the same host environment as your program; in that case, the debugging target is specified as a side effect when you use the `file' or `core' commands. When you need more flexibility--for example, running GDB on a physically separate host, or controlling a standalone system over a serial port or a realtime system over a TCP/IP connection--you can use the `target' command to specify one of the target types configured for GDB (*note Commands for managing targets: Target Commands.). * Menu: * Active Targets:: Active targets * Target Commands:: Commands for managing targets * Remote:: Remote debugging File: gdb.info, Node: Active Targets, Next: Target Commands, Up: Targets Active targets ============== There are three classes of targets: processes, core files, and executable files. GDB can work concurrently on up to three active targets, one in each class. This allows you to (for example) start a process and inspect its activity without abandoning your work on a core file. For example, if you execute `gdb a.out', then the executable file `a.out' is the only active target. If you designate a core file as well--presumably from a prior run that crashed and coredumped--then GDB has two active targets and will use them in tandem, looking first in the corefile target, then in the executable file, to satisfy requests for memory addresses. (Typically, these two classes of target are complementary, since core files contain only a program's read-write memory--variables and so on--plus machine status, while executable files contain only the program text and initialized data.) When you type `run', your executable file becomes an active process target as well. When a process target is active, all GDB commands requesting memory addresses refer to that target; addresses in an active core file or executable file target are obscured while the process target is active. Use the `core-file' and `exec-file' commands to select a new core file or executable target (*note Commands to specify files: Files.). To specify as a target a process that is already running, use the `attach' command (*note Debugging an already-running process: Attach.). File: gdb.info, Node: Target Commands, Next: Remote, Prev: Active Targets, Up: Targets Commands for managing targets ============================= `target TYPE PARAMETERS' Connects the GDB host environment to a target machine or process. A target is typically a protocol for talking to debugging facilities. You use the argument TYPE to specify the type or protocol of the target machine. Further PARAMETERS are interpreted by the target protocol, but typically include things like device names or host names to connect with, process numbers, and baud rates. The `target' command will not repeat if you press RET again after executing the command. `help target' Displays the names of all targets available. To display targets currently selected, use either `info target' or `info files' (*note Commands to specify files: Files.). `help target NAME' Describe a particular target, including any parameters necessary to select it. Here are some common targets (available, or not, depending on the GDB configuration): `target exec PROGRAM' An executable file. `target exec PROGRAM' is the same as `exec-file PROGRAM'. `target core FILENAME' A core dump file. `target core FILENAME' is the same as `core-file FILENAME'. `target remote DEV' Remote serial target in GDB-specific protocol. The argument DEV specifies what serial device to use for the connection (e.g. `/dev/ttya'). *Note Remote debugging: Remote. `target sim' CPU simulator. *Note Simulated CPU Target: Simulator. `target udi KEYWORD' Remote AMD29K target, using the AMD UDI protocol. The KEYWORD argument specifies which 29K board or simulator to use. *Note GDB and the UDI protocol for AMD29K: UDI29K Remote. `target amd-eb DEV SPEED PROG' Remote PC-resident AMD EB29K board, attached over serial lines. dEV is the serial device, as for `target remote'; SPEED allows you to specify the linespeed; and PROG is the name of the program to be debugged, as it appears to DOS on the PC. *Note GDB with a remote EB29K: EB29K Remote. `target hms' A Hitachi SH, H8/300, or H8/500 board, attached via serial line to your host. Use special commands `device' and `speed' to control the serial line and the communications speed used. *Note GDB and Hitachi Microprocessors: Hitachi Remote. `target nindy DEVICENAME' An Intel 960 board controlled by a Nindy Monitor. DEVICENAME is the name of the serial device to use for the connection, e.g. `/dev/ttya'. *Note GDB with a remote i960 (Nindy): i960-Nindy Remote. `target st2000 DEV SPEED' A Tandem ST2000 phone switch, running Tandem's STDBUG protocol. dEV is the name of the device attached to the ST2000 serial line; SPEED is the communication line speed. The arguments are not used if GDB is configured to connect to the ST2000 using TCP or Telnet. *Note GDB with a Tandem ST2000: ST2000 Remote. `target vxworks MACHINENAME' A VxWorks system, attached via TCP/IP. The argument MACHINENAME is the target system's machine name or IP address. *Note GDB and VxWorks: VxWorks Remote. Different targets are available on different configurations of GDB; your configuration may have more or fewer targets. File: gdb.info, Node: Remote, Prev: Target Commands, Up: Targets Remote debugging ================ If you are trying to debug a program running on a machine that cannot run GDB in the usual way, it is often useful to use remote debugging. For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger. Some configurations of GDB have special serial or TCP/IP interfaces to make this work with particular debugging targets. In addition, GDB comes with a generic serial protocol (specific to GDB, but not specific to any particular target system) which you can use if you write the remote stubs--the code that will run on the remote system to communicate with GDB. Other remote targets may be available in your configuration of GDB; use `help targets' to list them. * Menu: * Remote Serial:: GDB remote serial protocol * i960-Nindy Remote:: GDB with a remote i960 (Nindy) * UDI29K Remote:: GDB and the UDI protocol for AMD29K * EB29K Remote:: GDB with a remote EB29K * VxWorks Remote:: GDB and VxWorks * ST2000 Remote:: GDB with a Tandem ST2000 * Hitachi Remote:: GDB and Hitachi Microprocessors * MIPS Remote:: GDB and MIPS boards * Simulator:: Simulated CPU target File: gdb.info, Node: Remote Serial, Next: i960-Nindy Remote, Up: Remote The GDB remote serial protocol ------------------------------ To debug a program running on another machine (the debugging "target" machine), you must first arrange for all the usual prerequisites for the program to run by itself. For example, for a C program, you need 1. A startup routine to set up the C runtime environment; these usually have a name like `crt0'. The startup routine may be supplied by your hardware supplier, or you may have to write your own. 2. You probably need a C subroutine library to support your program's subroutine calls, notably managing input and output. 3. A way of getting your program to the other machine--for example, a download program. These are often supplied by the hardware manufacturer, but you may have to write your own from hardware documentation. The next step is to arrange for your program to use a serial port to communicate with the machine where GDB is running (the "host" machine). In general terms, the scheme looks like this: *On the host,* GDB already understands how to use this protocol; when everything else is set up, you can simply use the `target remote' command (*note Specifying a Debugging Target: Targets.). *On the target,* you must link with your program a few special-purpose subroutines that implement the GDB remote serial protocol. The file containing these subroutines is called a "debugging stub". The debugging stub is specific to the architecture of the remote machine; for example, use `sparc-stub.c' to debug programs on SPARC boards. These working remote stubs are distributed with GDB: `sparc-stub.c' For SPARC architectures. `m68k-stub.c' For Motorola 680x0 architectures. `i386-stub.c' For Intel 386 and compatible architectures. The `README' file in the GDB distribution may list other recently added stubs. * Menu: * Stub Contents:: What the stub can do for you * Bootstrapping:: What you must do for the stub * Debug Session:: Putting it all together * Protocol:: Outline of the communication protocol File: gdb.info, Node: Stub Contents, Next: Bootstrapping, Up: Remote Serial What the stub can do for you ---------------------------- The debugging stub for your architecture supplies these three subroutines: `set_debug_traps' This routine arranges for `handle_exception' to run when your program stops. You must call this subroutine explicitly near the beginning of your program. `handle_exception' This is the central workhorse, but your program never calls it explicitly--the setup code arranges for `handle_exception' to run when a trap is triggered. `handle_exception' takes control when your program stops during execution (for example, on a breakpoint), and mediates communications with GDB on the host machine. This is where the communications protocol is implemented; `handle_exception' acts as the GDB representative on the target machine; it begins by sending summary information on the state of your program, then continues to execute, retrieving and transmitting any information GDB needs, until you execute a GDB command that makes your program resume; at that point, `handle_exception' returns control to your own code on the target machine. `breakpoint' Use this auxiliary subroutine to make your program contain a breakpoint. Depending on the particular situation, this may be the only way for GDB to get control. For instance, if your target machine has some sort of interrupt button, you won't need to call this; pressing the interrupt button will transfer control to `handle_exception'--in efect, to GDB. On some machines, simply receiving characters on the serial port may also trigger a trap; again, in that situation, you don't need to call `breakpoint' from your own program--simply running `target remote' from the host GDB session will get control. Call `breakpoint' if none of these is true, or if you simply want to make certain your program stops at a predetermined point for the start of your debugging session. File: gdb.info, Node: Bootstrapping, Next: Debug Session, Prev: Stub Contents, Up: Remote Serial What you must do for the stub ----------------------------- The debugging stubs that come with GDB are set up for a particular chip architecture, but they have no information about the rest of your debugging target machine. To allow the stub to work, you must supply these special low-level subroutines: `int getDebugChar()' Write this subroutine to read a single character from the serial port. It may be identical to `getchar' for your target system; a different name is used to allow you to distinguish the two if you wish. `void putDebugChar(int)' Write this subroutine to write a single character to the serial port. It may be identical to `putchar' for your target system; a different name is used to allow you to distinguish the two if you wish. `void flush_i_cache()' Write this subroutine to flush the instruction cache, if any, on your target machine. If there is no instruction cache, this subroutine may be a no-op. On target machines that have instruction caches, GDB requires this function to make certain that the state of your program is stable. You must also make sure this library routine is available: `void *memset(void *, int, int)' This is the standard library function `memset' that sets an area of memory to a known value. If you have one of the free versions of `libc.a', `memset' can be found there; otherwise, you must either obtain it from your hardware manufacturer, or write your own. If you do not use the GNU C compiler, you may need other standard library subroutines as well; this will vary from one stub to another, but in general the stubs are likely to use any of the common library subroutines which `gcc' generates as inline code. File: gdb.info, Node: Debug Session, Next: Protocol, Prev: Bootstrapping, Up: Remote Serial Putting it all together ----------------------- In summary, when your program is ready to debug, you must follow these steps. 1. Make sure you have the supporting low-level routines: `getDebugChar', `putDebugChar', `flush_i_cache', `memset'. 2. Insert these lines near the top of your program: set_debug_traps(); breakpoint(); 3. Compile and link together: your program, the GDB debugging stub for your target architecture, and the supporting subroutines. 4. Make sure you have a serial connection between your target machine and the GDB host, and identify the serial port used for this on the host. 5. Download your program to your target machine (or get it there by whatever means the manufacturer provides), and start it. 6. To start remote debugging, run GDB on the host machine, and specify as an executable file the program that is running in the remote machine. This tells GDB how to find your program's symbols and the contents of its pure text. Then establish communication using the `target remote' command. Its argument is the name of the device you're using to control the target machine. For example: target remote /dev/ttyb if the serial line is connected to the device named `/dev/ttyb'. Now you can use all the usual commands to examine and change data and to step and continue the remote program. To resume the remote program and stop debugging it, use the `detach' command. Whenever GDB is waiting for the remote program, if you type the interrupt character (often C-C), GDB attempts to stop the program. This may or may not succeed, depending in part on the hardware and the serial drivers the remote system uses. If you type the interrupt character once again, GDB displays this prompt: Interrupted while waiting for the program. Give up (and stop debugging it)? (y or n) If you type `y', GDB abandons the remote debugging session. (If you decide you want to try again later, you can use `target remote' again to connect once more.) If you type `n', GDB goes back to waiting. File: gdb.info, Node: Protocol, Prev: Debug Session, Up: Remote Serial Outline of the communication protocol ------------------------------------- The stub files provided with GDB implement the target side of the communication protocol, and the GDB side is implemented in the GDB source file `remote.c'. Normally, you can simply allow these subroutines to communicate, and ignore the details. (If you're implementing your own stub file, you can still ignore the details: start with one of the existing stub files. `sparc-stub.c' is the best organized, and therefore the easiest to read.) However, there may be occasions when you need to know something about the protocol--for example, if there is only one serial port to your target machine, you might want your program to do something special if it recognizes a packet meant for GDB. All GDB commands and responses (other than acknowledgements, which are single characters) are sent as a packet which includes a checksum. A packet is introduced with the character `$', and ends with the character `#' followed by a two-digit checksum: $PACKET INFO#CHECKSUM CHECKSUM is computed as the modulo 256 sum of the PACKET INFO characters. When either the host or the target machine receives a packet, the first response expected is an acknowledgement: a single character, either `+' (to indicate the package was received correctly) or `-' (to request retransmission). The host (GDB) sends commands, and the target (the debugging stub incorporated in your program) sends data in response. The target also sends data when your program stops. Command packets are distinguished by their first character, which identifies the kind of command. These are the commands currently supported: `g' Requests the values of CPU registers. `G' Sets the values of CPU registers. `mADDR,COUNT' Read COUNT bytes at location ADDR. `MADDR,COUNT:...' Write COUNT bytes at location ADDR. `c' `cADDR' Resume execution at the current address (or at ADDR if supplied). `s' `sADDR' Step the target program for one instruction, from either the current program counter or from ADDR if supplied. `k' Kill the target program. `?' Report the most recent signal. To allow you to take advantage of the GDB signal handling commands, one of the functions of the debugging stub is to report CPU traps as the corresponding POSIX signal values. If you have trouble with the serial connection, you can use the command `set remotedebug'. This makes GDB report on all packets sent back and forth across the serial line to the remote machine. The packet-debugging information is printed on the GDB standard output stream. `set remotedebug off' turns it off, and `show remotedebug' will show you its current state. File: gdb.info, Node: i960-Nindy Remote, Next: UDI29K Remote, Prev: Remote Serial, Up: Remote GDB with a remote i960 (Nindy) ------------------------------ "Nindy" is a ROM Monitor program for Intel 960 target systems. When GDB is configured to control a remote Intel 960 using Nindy, you can tell GDB how to connect to the 960 in several ways: * Through command line options specifying serial port, version of the Nindy protocol, and communications speed; * By responding to a prompt on startup; * By using the `target' command at any point during your GDB session. *Note Commands for managing targets: Target Commands. * Menu: * Nindy Startup:: Startup with Nindy * Nindy Options:: Options for Nindy * Nindy Reset:: Nindy reset command File: gdb.info, Node: Nindy Startup, Next: Nindy Options, Up: i960-Nindy Remote Startup with Nindy ------------------ If you simply start `gdb' without using any command-line options, you are prompted for what serial port to use, *before* you reach the ordinary GDB prompt: Attach /dev/ttyNN -- specify NN, or "quit" to quit: Respond to the prompt with whatever suffix (after `/dev/tty') identifies the serial port you want to use. You can, if you choose, simply start up with no Nindy connection by responding to the prompt with an empty line. If you do this and later wish to attach to Nindy, use `target' (*note Commands for managing targets: Target Commands.). File: gdb.info, Node: Nindy Options, Next: Nindy Reset, Prev: Nindy Startup, Up: i960-Nindy Remote Options for Nindy ----------------- These are the startup options for beginning your GDB session with a Nindy-960 board attached: `-r PORT' Specify the serial port name of a serial interface to be used to connect to the target system. This option is only available when GDB is configured for the Intel 960 target architecture. You may specify PORT as any of: a full pathname (e.g. `-r /dev/ttya'), a device name in `/dev' (e.g. `-r ttya'), or simply the unique suffix for a specific `tty' (e.g. `-r a'). `-O' (An uppercase letter "O", not a zero.) Specify that GDB should use the "old" Nindy monitor protocol to connect to the target system. This option is only available when GDB is configured for the Intel 960 target architecture. *Warning:* if you specify `-O', but are actually trying to connect to a target system that expects the newer protocol, the connection fails, appearing to be a speed mismatch. GDB repeatedly attempts to reconnect at several different line speeds. You can abort this process with an interrupt. `-brk' Specify that GDB should first send a `BREAK' signal to the target system, in an attempt to reset it, before connecting to a Nindy target. *Warning:* Many target systems do not have the hardware that this requires; it only works with a few boards. The standard `-b' option controls the line speed used on the serial port. File: gdb.info, Node: Nindy Reset, Prev: Nindy Options, Up: i960-Nindy Remote Nindy reset command ------------------- `reset' For a Nindy target, this command sends a "break" to the remote target system; this is only useful if the target has been equipped with a circuit to perform a hard reset (or some other interesting action) when a break is detected. File: gdb.info, Node: UDI29K Remote, Next: EB29K Remote, Prev: i960-Nindy Remote, Up: Remote GDB and the UDI protocol for AMD29K ----------------------------------- GDB supports AMD's UDI ("Universal Debugger Interface") protocol for debugging the a29k processor family. To use this configuration with AMD targets running the MiniMON monitor, you need the program `MONTIP', available from AMD at no charge. You can also use GDB with the UDI conformant a29k simulator program `ISSTIP', also available from AMD. `target udi KEYWORD' Select the UDI interface to a remote a29k board or simulator, where KEYWORD is an entry in the AMD configuration file `udi_soc'. This file contains keyword entries which specify parameters used to connect to a29k targets. If the `udi_soc' file is not in your working directory, you must set the environment variable `UDICONF' to its pathname. File: gdb.info, Node: EB29K Remote, Next: VxWorks Remote, Prev: UDI29K Remote, Up: Remote GDB with a remote EB29K ----------------------- To use GDB from a Unix system to run programs on AMD's EB29K board in a PC, you must first connect a serial cable between the PC and a serial port on the Unix system. In the following, we assume you've hooked the cable between the PC's `COM1' port and `/dev/ttya' on the Unix system. * Menu: * Comms (EB29K):: Communications setup * gdb-EB29K:: EB29K cross-debugging * Remote Log:: Remote log File: gdb.info, Node: Comms (EB29K), Next: gdb-EB29K, Up: EB29K Remote Communications setup -------------------- The next step is to set up the PC's port, by doing something like this in DOS on the PC: C:\> MODE com1:9600,n,8,1,none This example--run on an MS DOS 4.0 system--sets the PC port to 9600 bps, no parity, eight data bits, one stop bit, and no "retry" action; you must match the communications parameters when establishing the Unix end of the connection as well. To give control of the PC to the Unix side of the serial line, type the following at the DOS console: C:\> CTTY com1 (Later, if you wish to return control to the DOS console, you can use the command `CTTY con'--but you must send it over the device that had control, in our example over the `COM1' serial line). From the Unix host, use a communications program such as `tip' or `cu' to communicate with the PC; for example, cu -s 9600 -l /dev/ttya The `cu' options shown specify, respectively, the linespeed and the serial port to use. If you use `tip' instead, your command line may look something like the following: tip -9600 /dev/ttya Your system may require a different name where we show `/dev/ttya' as the argument to `tip'. The communications parameters, including which port to use, are associated with the `tip' argument in the "remote" descriptions file--normally the system table `/etc/remote'. Using the `tip' or `cu' connection, change the DOS working directory to the directory containing a copy of your 29K program, then start the PC program `EBMON' (an EB29K control program supplied with your board by AMD). You should see an initial display from `EBMON' similar to the one that follows, ending with the `EBMON' prompt `#'-- C:\> G: G:\> CD \usr\joe\work29k G:\USR\JOE\WORK29K> EBMON Am29000 PC Coprocessor Board Monitor, version 3.0-18 Copyright 1990 Advanced Micro Devices, Inc. Written by Gibbons and Associates, Inc. Enter '?' or 'H' for help PC Coprocessor Type = EB29K I/O Base = 0x208 Memory Base = 0xd0000 Data Memory Size = 2048KB Available I-RAM Range = 0x8000 to 0x1fffff Available D-RAM Range = 0x80002000 to 0x801fffff PageSize = 0x400 Register Stack Size = 0x800 Memory Stack Size = 0x1800 CPU PRL = 0x3 Am29027 Available = No Byte Write Available = Yes # ~. Then exit the `cu' or `tip' program (done in the example by typing `~.' at the `EBMON' prompt). `EBMON' will keep running, ready for GDB to take over. For this example, we've assumed what is probably the most convenient way to make sure the same 29K program is on both the PC and the Unix system: a PC/NFS connection that establishes "drive `G:'" on the PC as a file system on the Unix host. If you do not have PC/NFS or something similar connecting the two systems, you must arrange some other way--perhaps floppy-disk transfer--of getting the 29K program from the Unix system to the PC; GDB will *not* download it over the serial line.