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Published by the Free Software Foundation
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The purpose of a debugger such as GDB 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.
GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:
You can use GDB to debug programs written in C or C++. For more information, see section C and C++.
Support for Modula-2 and Chill is partial. For information on Modula-2, see section Modula-2. There is no further documentation on Chill yet.
Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. GDB does not support entering expressions, printing values, or similar features using Pascal syntax.
GDB can be used to debug programs written in Fortran, although it does not yet support entering expressions, printing values, or similar features using Fortran syntax. It may be necessary to refer to some variables with a trailing underscore.
GDB 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 cannot take these freedoms away from anyone else.
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, to 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: Stan Shebs (release 4.14), Fred Fish (releases 4.13, 4.12, 4.11, 4.10, and 4.9), Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4), John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2, 3.1, and 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 Peter 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 4 uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project of David V. 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.
Brent Benson of Harris Computer Systems contributed DWARF 2 support.
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. Jeff Law contributed HP PA and SOM support. Keith Packard contributed NS32K support. Doug Rabson contributed Acorn Risc Machine support. Bob Rusk contributed Harris Nighthawk CX-UX 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, Wind River Systems, AMD, and ARM contributed remote debugging modules for the i960, VxWorks, A29K UDI, and RDI targets, respectively.
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, the Modula-2 support, and contributed the Languages chapter of this manual.
Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the command-completion support to cover C++ overloaded symbols.
Hitachi America, Ltd. sponsored the support for Hitachi microprocessors.
Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware watchpoints.
Michael Snyder added support for tracepoints.
Stu Grossman wrote gdbserver.
Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly innumerable bug fixes and cleanups throughout GDB.
Cygnus Solutions has sponsored GDB maintenance and much of its development since 1991.
You can use this manual at your leisure to read all about GDB. However, a handful of commands are enough to get started using the debugger. This chapter illustrates those commands.
In this sample session, we emphasize user input like this: input, to make it easier to pick out from the surrounding output.
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
definition within another stop working. In the following short m4
session, we define a macro foo which expands to 0000; we
then use the m4 built-in 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 us use GDB to try to see what is going on.
$ gdb 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 , Copyright 1995 Free Software Foundation, Inc... (gdb)
GDB 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 now tell GDB to use a narrower display width than usual, so that examples fit in this manual.
(gdb) set width 70
We need to see how the m4 built-in changequote works.
Having looked at the source, we know the relevant subroutine is
m4_changequote, so we set a breakpoint there with the GDB
break command.
(gdb) break m4_changequote Breakpoint 1 at 0x62f4: file builtin.c, line 879.
Using the run command, we start m4 running under GDB
control; as long as control does not reach the m4_changequote
subroutine, the program runs as usual:
(gdb) run Starting program: /work/Editorial/gdb/gnu/m4/m4 define(foo,0000) foo 0000
To trigger the breakpoint, we call changequote. GDB
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.
(gdb) 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.
(gdb) s
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
530 if (lquote != def_lquote)
The display that shows the subroutine where m4 is now
suspended (and its arguments) is called a stack frame display. It
shows a summary of the stack. We can use the backtrace
command (which can also be spelled bt), to see where we are
in the stack as a whole: the backtrace command displays a
stack frame for each active subroutine.
(gdb) 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
We 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.
(gdb) s 0x3b5c 532 if (rquote != def_rquote) (gdb) s 0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \ def_lquote : xstrdup(lq); (gdb) n 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup(rq); (gdb) n 538 len_lquote = strlen(rquote);
The last line displayed looks a little odd; we can examine the variables
lquote and rquote to see if they are in fact the new left
and right quotes we specified. We use the command p
(print) to see their values.
(gdb) p lquote $1 = 0x35d40 "<QUOTE>" (gdb) p rquote $2 = 0x35d50 "<UNQUOTE>"
lquote and rquote are indeed the new left and right quotes.
To look at some context, we can display ten lines of source
surrounding the current line with the l (list) command.
(gdb) 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 us step past the two lines that set len_lquote and
len_rquote, and then examine the values of those variables.
(gdb) n 539 len_rquote = strlen(lquote); (gdb) n 540 } (gdb) p len_lquote $3 = 9 (gdb) 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. We can set them to better values using
the p command, since it can print the value of
any expression--and that expression can include subroutine calls and
assignments.
(gdb) p len_lquote=strlen(lquote) $5 = 7 (gdb) p len_rquote=strlen(rquote) $6 = 9
Is that enough to fix 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:
(gdb) 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 allow m4 exit by giving it an EOF as input:
C-d Program exited normally.
The message `Program exited normally.' is from GDB; it
indicates m4 has finished executing. We can end our GDB
session with the GDB quit command.
(gdb) quit
This chapter discusses how to start GDB, and how to get out of it. The essentials are:
Invoke GDB by running the program gdb. Once started,
GDB reads commands from the terminal until you tell it to exit.
You can also run gdb with a variety of arguments and options,
to specify more of your debugging environment at the outset.
The command-line options described here are designed to cover a variety of situations; in some environments, some of these options may effectively be unavailable.
The most usual way to start GDB is with one argument, specifying an executable program:
gdb program
You can also start with both an executable program and a core file specified:
gdb program core
You can, instead, specify a process ID as a second argument, if you want to debug a running process:
gdb program 1234
would attach GDB to process 1234 (unless you also have a file
named `1234'; GDB does check for a core file first).
Taking advantage of the second command-line argument requires a fairly complete operating system; when you use GDB as a remote debugger attached to a bare board, there may not be any notion of "process", and there is often no way to get a core dump.
You can run gdb without printing the front material, which describes
GDB's non-warranty, by specifying -silent:
gdb -silent
You can further control how GDB starts up by using command-line options. GDB itself can remind you of the options available.
Type
gdb -help
to display all available options and briefly describe their use (`gdb -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.
When GDB starts, it reads any arguments other than options as specifying an executable file and core file (or process ID). This is the same as if the arguments were specified by the `-se' and `-c' options respectively. (GDB reads the first argument that does not have an associated option flag as equivalent to the `-se' option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the `-c' option followed by that argument.)
Many options have both long and short forms; both are shown in the following list. GDB also recognizes the long forms 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
-exec file
-e file
-se file
-core file
-c file
-c number
attach command
(unless there is a file in core-dump format named number, in which
case `-c' specifies that file as a core dump to read).
-command file
-x file
-directory directory
-d directory
-m
-mapped
mmap
system call, you can use this option
to have GDB write the symbols from your
program into a reusable file in the current directory. If the program you are debugging is
called `/tmp/fred', the mapped symbol file is `./fred.syms'.
Future GDB debugging sessions notice the presence of this file,
and can quickly map in symbol information from it, rather than reading
the symbol table from the executable program.
The `.syms' file is specific to the host machine where GDB
is run. It holds an exact image of the internal GDB symbol
table. It cannot be shared across multiple host platforms.
-r
-readnow
The -mapped and -readnow options are typically combined in
order to build a `.syms' file that contains complete symbol
information. (See section Commands to specify files, for information
a `.syms' file for future use is:
gdb -batch -nx -mapped -readnow programname
You can run GDB in various alternative modes--for example, in batch mode or quiet mode.
-nx
-n
-quiet
-q
-batch
0 after processing all the
command files specified with `-x' (and all commands from
initialization files, if not inhibited with `-n'). Exit with
nonzero status if an error occurs in executing the GDB commands
in the command files.
Batch mode may be useful for running GDB 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 GDB control terminates) is not issued when running in batch mode.
-cd directory
-fullname
-f
-b bps
-tty device
quit
quit command (abbreviated q), or
type an end-of-file character (usually C-d). If you do not supply
expression, GDB will terminate normally; otherwise it will
terminate using the result of expression as the error code.
An interrupt (often C-c) does not exit from GDB, but rather terminates the action of any GDB command that is in progress and returns to GDB command level. It is safe to type the interrupt character at any time because GDB does not allow it to take effect until a time when it is safe.
If you have been using GDB to control an attached process or
device, you can release it with the detach command
(see section Debugging an already-running process).
If you need to execute occasional shell commands during your
debugging session, there is no need to leave or suspend GDB; you can
just use the shell command.
shell command string
SHELL determines which
shell to run. Otherwise GDB uses /bin/sh.
The utility make is often needed in development environments.
You do not have to use the shell command for this purpose in
GDB:
make make-args
make program with the specified
arguments. This is equivalent to `shell make make-args'.
You can abbreviate a GDB command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain GDB commands by typing just RET. You can also use the TAB key to get GDB to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility).
A GDB 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.
GDB 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 GDB (typing just RET) means to
repeat the previous command. Certain commands (for example, run)
will not repeat this way; these are commands whose 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.
GDB 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 is easy to press one
RET too many in this situation, GDB disables command
repetition after any command that generates this sort of display.
Any text from a # to the end of the line is a comment; it does nothing. This is useful mainly in command files (see section Command files).
GDB can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you what the valid possibilities are for the next word in a command, at any time. This works for GDB commands, GDB subcommands, and the names of symbols in your program.
Press the TAB key whenever you want GDB to fill out the rest of a word. If there is only one possibility, GDB fills in the word, and waits for you to finish the command (or press RET to enter it). For example, if you type
(gdb) info bre TAB
GDB fills in the rest of the word `breakpoints', since that is
the only info subcommand beginning with `bre':
(gdb) info breakpoints
You can either press RET at this point, to run the info
breakpoints command, or backspace and enter something else, if
`breakpoints' does not look like the command you expected. (If you
were sure you wanted info breakpoints in the first place, you
might as well just type RET immediately after `info bre',
to exploit command abbreviations rather than command completion).
If there is more than one possibility for the next word when you press TAB, GDB sounds a bell. You can either supply more characters and try again, or just press TAB a second time; GDB displays all the possible completions for that word. For example, you might want to set a breakpoint on a subroutine whose name begins with `make_', but when you type b make_TAB GDB just sounds the bell. Typing TAB again displays all the function names in your program that begin with those characters, for example:
(gdb) b make_ TAB GDB sounds bell; press TAB again, to see: make_a_section_from_file make_environ make_abs_section make_function_type make_blockvector make_pointer_type make_cleanup make_reference_type make_command make_symbol_completion_list (gdb) b make_
After displaying the available possibilities, GDB copies your partial input (`b make_' in the example) so you can finish the command.
If you just want to see the list of alternatives in the first place, you can press M-? rather than pressing TAB twice. M-? means META ?. You can type this either by holding down a key designated as the META shift on your keyboard (if there is one) while typing ?, or as ESC followed by ?.
Sometimes the string you need, while logically a "word", may contain
parentheses or other characters that GDB normally excludes from its
notion of a word. To permit word completion to work in this situation,
you may enclose words in ' (single quote marks) in GDB commands.
The most likely situation where you might need this is in typing the
name of a C++ function. This is because C++ allows function overloading
(multiple definitions of the same function, distinguished by argument
type). For example, when you want to set a breakpoint you may need to
distinguish whether you mean the version of name that takes an
int parameter, name(int), or the version that takes a
float parameter, name(float). To use the word-completion
facilities in this situation, type a single quote ' at the
beginning of the function name. This alerts GDB that it may need to
consider more information than usual when you press TAB or
M-? to request word completion:
(gdb) b 'bubble( M-? bubble(double,double) bubble(int,int) (gdb) b 'bubble(
In some cases, GDB can tell that completing a name requires using quotes. When this happens, GDB inserts the quote for you (while completing as much as it can) if you do not type the quote in the first place:
(gdb) b bub TAB GDB alters your input line to the following, and rings a bell: (gdb) b 'bubble(
In general, GDB can tell that a quote is needed (and inserts it) if you have not yet started typing the argument list when you ask for completion on an overloaded symbol.
You can always ask GDB itself for information on its commands,
using the command help.
help
h
help (abbreviated h) with no arguments to
display a short list of named classes of commands:
(gdb) 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. (gdb)
help class
status:
(gdb) 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. (gdb)
help command
help argument, GDB displays a
short paragraph on how to use that command.
complete args
complete args command lists all the possible completions
for the beginning of a command. Use args to specify the beginning of the
command you want completed. For example:
complete iresults in:
info inspect ignoreThis is intended for use by GNU Emacs.
In addition to help, you can use the GDB commands info
and show to inquire about the state of your program, or the state
of GDB 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. See section Index.
info
i) is for describing the state of your
program. For example, you can list the arguments given to your program
with info args, list the registers currently in use with info
registers, or list the breakpoints you have set with info breakpoints.
You can get a complete list of the info sub-commands with
help info.
set
set. For example, you can set the GDB prompt to a $-sign with
set prompt $.
show
info, show is for describing the state of
GDB 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 copying
show warranty
When you run a program under GDB, you must first generate debugging information when you compile it. You may start GDB with its arguments, if any, in an environment of your choice. You may redirect your program's input and output, debug an already running process, or kill a child process.
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.
GCC, 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 your program is correct, but there is no sense in pushing your luck.
When you debug a program compiled with `-g -O', remember that the optimizer is rearranging your code; the debugger shows you what is really there. Do not be too surprised when the execution path does not exactly match your source file! An extreme example: if you define a variable, but never use it, GDB never sees that variable--because the compiler optimizes it out of existence.
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 to us as a bug (including a test case!).
Older versions of the GNU C compiler permitted a variant option `-gg' for debugging information. GDB no longer supports this format; if your GNU C compiler has this option, do not use it.
run
r
run command to start your program under GDB. You must
first specify the program name
(except on VxWorks)
with an argument to GDB (see section Getting In and Out of GDB), or by using the file or exec-file
command (see section Commands to specify files).
If you are running your program in an execution environment that
supports processes, run creates an inferior process and makes
that process run your program. (In environments without processes,
run jumps to the start of your program.)
The execution of a program is affected by certain information it receives from its superior. GDB provides ways to specify this information, which you must do before starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into four categories:
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.
set environment and unset
environment to change parts of the environment that affect
your program. See section Your program's environment.
cd command in GDB.
See section Your program's working directory.
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 cannot use
pipes to pass the output of the program you are debugging to another
program; if you attempt this, GDB 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
stopped, you may call functions in your program, using the print
or call commands. See section Examining Data.
If the modification time of your symbol file has changed since the last time GDB read its symbols, GDB discards its symbol table, and reads it again. When it does this, GDB tries to retain your current breakpoints.
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 your program.
Your SHELL environment variable (if it exists) specifies what
shell GDB uses. If you do not define SHELL,
GDB 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
set args has no arguments, run executes 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
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 your program with a modified environment without having to start GDB over again.
path directory
PATH environment variable
(the search path for executables), for both GDB 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 is searched sooner.
You can use the string `$cwd' to refer to whatever is the current
working directory at the time GDB searches the path. If you
use `.' instead, it refers to the directory where you executed the
path command. GDB replaces `.' in the
directory argument (with the current path) before adding
directory to the search path.
show paths
PATH
environment variable).
show environment [varname]
environment as env.
set environment varname [=] value
set env USER = footells 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
unset environment removes the variable from the environment,
rather than assigning it an empty value.
Warning: GDB runs your program using the shell indicated
by your SHELL environment variable if it exists (or
/bin/sh if not). If your SHELL variable names a shell
that runs an initialization file--such as `.cshrc' for C-shell, or
`.bashrc' for BASH--any variables you set in that file affect
your program. You may wish to move setting of environment variables to
files that are only run when you sign on, such as `.login' or
`.profile'.
Each time you start your program with run, it inherits its
working directory from the current working directory of GDB.
The GDB working directory is initially whatever it inherited
from its parent process (typically the shell), but you can specify a new
working directory in GDB with the cd command.
The GDB working directory also serves as a default for the commands that specify files for GDB to operate on. See section Commands to specify files.
cd directory
pwd
By default, the program you run under GDB does input and output to the same terminal that GDB uses. GDB 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
You can redirect your program's input and/or output using shell
redirection with the run command. For example,
run > outfile
starts your program, diverting its output to the file `outfile'.
Another way to specify where your 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 GDB still comes from your terminal.
attach process-id
info files shows 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 does not repeat if you press RET a second time after
executing the command.
To use attach, your program must be running in an environment
which supports processes; for example, attach does not work for
programs on bare-board targets that lack an operating system. You must
also have permission to send the process a signal.
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 GDB does after arranging to debug the specified
process is to stop it. You can examine and modify an attached process
with all the GDB 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 GDB to the process.
detach
detach command to release it from GDB control. Detaching
the process continues its execution. After the detach command,
that process and GDB become completely independent once more, and you
are ready to attach another process or start one with run.
detach does not repeat if you press RET again after
executing the command.
If you exit GDB or use the run command while you have an
attached process, you kill that process. By default, GDB asks
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).
kill
This command is useful if you wish to debug a core dump instead of a running process. GDB ignores any core dump file while your program is running.
On some operating systems, a program cannot be executed outside GDB
while you have breakpoints set on it inside GDB. You can use the
kill command in this situation to permit running your program
outside the debugger.
The kill command is also useful if you wish to recompile and
relink your 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, GDB notices that the file has changed, and
reads the symbol table again (while trying to preserve your current
breakpoint settings).
Some operating systems provide a facility called `/proc' that can
be used to examine the image of a running process using file-system
subroutines. If GDB is configured for an operating system with this
facility, the command info proc is available to report on several
kinds of information about the process running your program.
info proc works only on SVR4 systems that support procfs.
info proc
info proc mappings
info proc times
info proc id
info proc status
info proc all
In some operating systems, a single program may have more than one thread of execution. The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes--except that they share one address space (that is, they can all examine and modify the same variables). On the other hand, each thread has its own registers and execution stack, and perhaps private memory.
GDB provides these facilities for debugging multi-thread programs:
Warning: These facilities are not yet available on every GDB configuration where the operating system supports threads. If your GDB does not support threads, these commands have no effect. For example, a system without thread support shows no output from `info threads', and always rejects the
threadcommand, like this:(gdb) info threads (gdb) thread 1 Thread ID 1 not known. Use the "info threads" command to see the IDs of currently known threads.
The GDB thread debugging facility allows you to observe all threads while your program runs--but whenever GDB takes control, one thread in particular is always the focus of debugging. This thread is called the current thread. Debugging commands show program information from the perspective of the current thread.
Whenever GDB detects a new thread in your program, it displays the target system's identification for the thread with a message in the form `[New systag]'. systag is a thread identifier whose form varies depending on the particular system. For example, on LynxOS, you might see
[New process 35 thread 27]
when GDB notices a new thread. In contrast, on an SGI system, the systag is simply something like `process 368', with no further qualifier.
For debugging purposes, GDB associates its own thread number--always a single integer--with each thread in your program.
info threads
(gdb) info threads
3 process 35 thread 27 0x34e5 in sigpause ()
2 process 35 thread 23 0x34e5 in sigpause ()
* 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8)
at threadtest.c:68
thread threadno
(gdb) thread 2 [Switching to process 35 thread 23] 0x34e5 in sigpause ()As with the `[New ...]' message, the form of the text after `Switching to' depends on your system's conventions for identifying threads.
thread apply [threadno] [all] args
thread apply command allows you to apply a command to one or
more threads. Specify the numbers of the threads that you want affected
with the command argument threadno. threadno is the internal
GDB thread number, as shown in the first field of the `info
threads' display. To apply a command to all threads, use
thread apply all args.
Whenever GDB stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. GDB alerts you to the context switch with a message of the form `[Switching to systag]' to identify the thread.
See section Stopping and starting multi-thread programs, for more information about how GDB behaves when you stop and start programs with multiple threads.
See section Setting watchpoints, for information about watchpoints in programs with multiple threads.
GDB has no special support for debugging programs which create
additional processes using the fork function. When a program
forks, GDB will continue to debug the parent process and the
child process will run unimpeded. If you have set a breakpoint in any
code which the child then executes, the child will get a SIGTRAP
signal which (unless it catches the signal) will cause it to terminate.
However, if you want to debug the child process there is a workaround
which isn't too painful. Put a call to sleep in the code which
the child process executes after the fork. It may be useful to sleep
only if a certain environment variable is set, or a certain file exists,
so that the delay need not occur when you don't want to run GDB
on the child. While the child is sleeping, use the ps program to
get its process ID. Then tell GDB (a new invocation of
GDB if you are also debugging the parent process) to attach to
the child process (see section Debugging an already-running process). From that point on you can debug
the child process just like any other process which you attached to.
The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and find out why.
Inside GDB, your program may stop for any of several reasons, such
as
a signal,
a breakpoint, or reaching a new line after a GDB
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 GDB provide ample
explanation of the status of your program--but you can also explicitly
request this information at any time.
info program
A breakpoint makes your program stop whenever a certain point in
the program is reached. For each breakpoint, you can add
conditions to control in finer detail whether your program stops.
You can set breakpoints with the break command and its variants
(see section Setting breakpoints), to specify the place where
your 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).
In SunOS 4.x, SVR4, and Alpha OSF/1 configurations, you can now set breakpoints in shared libraries before the executable is run.
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.
You can arrange to have values from your program displayed automatically whenever GDB stops at a breakpoint. See section Automatic display.
GDB assigns a number to each breakpoint or watchpoint when you create it; 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 your program until you enable it again.
Breakpoints are set with the break command (abbreviated
b). The debugger convenience variable `$bpnum' records the
number of the breakpoints you've set most recently; see section Convenience variables, for a discussion of what you can do with
convenience variables.
You have several ways to say where the breakpoint should go.
break function
break +offset
break -offset
break linenum
break filename:linenum
break filename:function
break *address
break
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 makes your program 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 does not leave an active breakpoint. If you use
break without an argument in the innermost frame, GDB stops
the next time it reaches the current location; this may be useful
inside loops.
GDB 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 your program stopped.
break ... if cond
tbreak args
break command, and the breakpoint is set in the same
way, but the breakpoint is automatically deleted after the first time your
program stops there. See section Disabling breakpoints.
hbreak args
break command and the breakpoint is set in the same way, but the
breakpoint requires hardware support and some target hardware may not
have this support. The main purpose of this is EPROM/ROM code
debugging, so you can set a breakpoint at an instruction without
changing the instruction. This can be used with the new trap-generation
provided by SPARClite DSU. DSU will generate traps when a program accesses
some date or instruction address that is assigned to the debug registers.
However the hardware breakpoint registers can only take two data breakpoints,
and GDB will reject this command if more than two are used.
Delete or disable usused hardware breakpoints before setting
new ones. See section Break conditions.
thbreak args
hbreak command and the breakpoint is set in
the same way. However, like the tbreak command,
the breakpoint is automatically deleted after the
first time your program stops there. Also, like the hbreak
command, the breakpoint requires hardware support and some target hardware
may not have this support. See section Disabling breakpoints.
Also See section Break conditions.
rbreak regex
break command. You can
delete them, disable them, or make them conditional the same way as any
other breakpoint.
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]
info watchpoints [n]
info break shows the condition on
the line following the affected breakpoint; breakpoint commands, if any,
are listed after that.
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).
info break now displays a count of the number of times the
breakpoint has been hit. This is especially useful in conjunction with
the ignore command. You can ignore a large number of breakpoint
hits, look at the breakpoint info to see how many times the
breakpoint was hit, and then run again, ignoring one less than that
number. This will get you quickly to the last hit of that breakpoint.
GDB allows you to set any number of breakpoints at the same place in your program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (see section Break conditions).
GDB itself sometimes sets breakpoints in your program for special
purposes, such as proper handling of longjmp (in C programs).
These internal breakpoints are assigned negative numbers, starting with
-1; `info breakpoints' does not display them.
You can see these breakpoints with the GDB maintenance command `maint info breakpoints'.
maint info breakpoints
breakpoint
watchpoint
longjmp
longjmp calls.
longjmp resume
longjmp.
until
until command.
finish
finish command.
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 be well worth it to catch errors where you have no clue what part of your program is the culprit.
watch expr
watch command.
However the hardware breakpoint registers can only take two data watchpoints,
and both watchpoints must be the same kind. For example, you can set two
watchpoints with watch commands, two with rwatch
commands, or two with awatch commands, but you cannot set one
watchpoint with one command and the other with a different command.
will reject the command if you try to mix watchpoints.
Delete or disable unused watchpoint commands before setting new ones.
rwatch expr
rwatch
command.
awatch expr
awatch command.
info watchpoints
info break.
Warning: in multi-thread programs, watchpoints have only limited usefulness. With the current watchpoint implementation, GDB can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression.
Some languages, such as GNU C++, implement exception handling. You can use GDB to examine what caused your program to raise an exception, and to list the exceptions your program is prepared to handle at a given point in time.
catch exceptions
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 GDB:
Sometimes catch is not the best way to debug exception handling:
if you need to know exactly where an exception is raised, it is 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 your program when any of a number of exceptions are raised.
It is often necessary to eliminate a breakpoint or watchpoint once it has done its job and you no longer want your 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 your 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. GDB automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.
clear
clear function
clear filename:function
clear linenum
clear filename:linenum
delete [breakpoints] [bnums...]
set confirm off). You
can abbreviate this command as d.
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
do not know which numbers to use.
A breakpoint or watchpoint can have any of four different states of enablement:
break command starts out in this state.
tbreak command starts out in
this state.
You can use the following commands to enable or disable breakpoints and watchpoints:
disable [breakpoints] [bnums...]
disable as dis.
enable [breakpoints] [bnums...]
enable [breakpoints] once bnums...
enable [breakpoints] delete bnums...
Except 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 does not change the state of your other
breakpoints; see section Continuing and stepping.)
The simplest sort of breakpoint breaks every time your 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 your program reaches it, and your 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 can 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, GDB might see the other breakpoint first and stop your 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 does not
recognize the if keyword; condition is the only way to
impose a further condition on a watchpoint.
condition bnum expression
condition, GDB
checks expression immediately for syntactic correctness, and to
determine whether symbols in it have referents in the context of your
breakpoint.
GDB does
not actually evaluate expression at the time the condition
command is given, however. See section Expressions.
condition bnum
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 your 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 does not stop the next n times your program reaches it.
ignore bnum count
continue to resume execution of your program from a
breakpoint, you can specify an ignore count directly as an argument to
continue, rather than using ignore. See section Continuing and stepping.
If a breakpoint has a positive ignore count and a condition, the
condition is not checked. Once the ignore count reaches zero,
GDB resumes checking the condition.
You could achieve the effect of the ignore count with a condition such
as `$foo-- <= 0' using a debugger convenience variable that
is decremented each time. See section Convenience variables.
You can give any breakpoint (or watchpoint) a series of commands to execute when your 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
end to terminate the commands.
To remove all commands from a breakpoint, type commands and
follow it immediately with 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 GDB command is disabled within a command-list.
You can use breakpoint commands to start your program up again. Simply
use the continue command, or step, or any other command
that resumes execution.
Any other commands in the command list, after a command that resumes
execution, are ignored. This is because any time you resume execution
(even with a simple next or step), you may encounter
another breakpoint--which could have its own command list, leading to
ambiguities about which list to execute.
If the first command you specify in a command list 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 none of the remaining commands print anything, you
see no sign that the breakpoint was reached. silent is
meaningful only at the beginning of a breakpoint command list.
The commands echo, output, and printf allow you to
print precisely controlled output, and 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.
break foo if x>0 commands silent printf "x is %d\n",x cont end
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 your 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
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 GDB where you want
a breakpoint. If you realize this is a problem, you can use
something like `break function(types)' to specify which
particular version of the function you want. Otherwise, GDB 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:
(gdb) 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. (gdb)
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, your program may stop even sooner, due to
a breakpoint or 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]
c [ignore-count]
fg [ignore-count]
ignore (see section Break conditions).
The argument ignore-count is meaningful only when your program
stopped due to a breakpoint. At other times, the argument to
continue is ignored.
The synonyms c and fg are provided purely for convenience,
and have exactly the same behavior as continue.
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 your program where a problem is believed to lie, run your 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
s.
TheWarning: If you use the
stepcommand while control is within a function that was compiled without debugging information, execution proceeds until control reaches a function that does have debugging information. Likewise, it will not step into a function which is compiled without debugging information. To step through functions without debugging information, use thestepicommand, described below.
step command now only stops at the first instruction of a
source line. This prevents the multiple stops that used to occur in
switch statements, for loops, etc. step continues to stop if a
function that has debugging information is called within the line.
Also, the step command now only enters a subroutine if there is line
number information for the subroutine. Otherwise it acts like the
next command. This avoids problems when using cc -gl
on MIPS machines. Previously, step entered subroutines if there
was any debugging information about the routine.
step count
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]
step, but function calls that appear within the line
of code are executed without stopping. Execution stops when control
reaches a different line of code at the original stack level that was
executing when you gave the next command. This command is abbreviated
n.
An argument count is a repeat count, as for step.
The next command now only stops at the first instruction of a
source line. This prevents the multiple stops that used to occur in
swtch statements, for loops, etc.
finish
return command (see section Returning from a function).
u
until
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 makes your program continue execution until it
exits the loop. In contrast, a next command at the end of a loop
simply steps back to the beginning of the loop, which forces you to step
through the next iteration.
until always stops your 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:
(gdb) f
#0 main (argc=4, argv=0xf7fffae8) at m4.c:206
206 expand_input();
(gdb) 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
break (see section Setting breakpoints). This form of the command uses breakpoints,
and hence is quicker than until without an argument.
stepi
si
step.
nexti
ni
next.
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 your program has
requested an alarm).
Some signals, including SIGALRM, are a normal part of the
functioning of your program. Others, such as SIGSEGV, indicate
errors; these signals are fatal (kill your program immediately) if the
program has not specified in advance some other way to handle the signal.
SIGINT does not indicate an error in your program, but it is normally
fatal so it can carry out the purpose of the interrupt: to kill the program.
GDB has the ability to detect any occurrence of a signal in your program. You can tell GDB in advance what to do for each kind of signal.
Normally, GDB is set up to ignore non-erroneous signals like SIGALRM
(so as not to interfere with their role in the functioning of your program)
but to stop your program immediately whenever an error signal happens.
You can change these settings with the handle command.
info signals
info handle is the new alias for info signals.
handle signal keywords...
The keywords allowed by the handle command can be abbreviated.
Their full names are:
nostop
stop
print keyword as well.
print
noprint
nostop keyword as well.
pass
nopass
When a signal stops your program, the signal is not visible until you
continue. Your program sees the signal then, if pass is in
effect for the signal in question at that time. In other words,
after GDB reports a signal, you can use the handle
command with pass or nopass to control whether your
program sees that signal when you continue.
You can also use the signal command to prevent your 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 your 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 your program would probably terminate immediately as
a result of the fatal signal once it saw the signal. To prevent this,
you can continue with `signal 0'. See section Giving your program a signal.
When your program has multiple threads (see section Debugging programs with multiple threads), you can choose whether to set breakpoints on all threads, or on a particular thread.
break linespec thread threadno
break linespec thread threadno if ...
thread qualifier on conditional breakpoints as
well; in this case, place `thread threadno' before the
breakpoint condition, like this:
(gdb) break frik.c:13 thread 28 if bartab > lim
Whenever your program stops under GDB for any reason, all threads of execution stop, not just the current thread. This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot.
Conversely, whenever you restart the program, all threads start
executing. This is true even when single-stepping with commands
like step or next.
In particular, GDB cannot single-step all threads in lockstep. Since thread scheduling is up to your debugging target's operating system (not controlled by GDB), other threads may execute more than one statement while the current thread completes a single step. Moreover, in general other threads stop in the middle of a statement, rather than at a clean statement boundary, when the program stops.
You might even find your program stopped in another thread after continuing or even single-stepping. This happens whenever some other thread runs into a breakpoint, a signal, or an exception before the first thread completes whatever you requested.
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, information about the call is generated. That information includes the location of the call in your program, the arguments of the call, and the local variables of the function being called. The information is saved in a block of data called a stack frame. The stack frames are allocated in a region of memory called the call stack.
When your program stops, the GDB commands for examining the stack allow you to see all of this information.
One of the stack frames is selected by GDB and many GDB commands refer implicitly to the selected frame. In particular, whenever you ask GDB for the value of a variable in your program, the value is found in the selected frame. There are special GDB commands to select whichever frame you are interested in. See section Selecting a frame.
When your program stops, GDB automatically selects the
currently executing frame and describes it briefly, similar to the
frame command (see section Information about a frame).
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 byte 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.
GDB 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 GDB to give you a way of designating stack frames in GDB commands.
Some compilers provide a way to compile functions so that they operate
without stack frames. (For example, the gcc option
`-fomit-frame-pointer' generates functions without a frame.)
This is occasionally done with heavily used library functions to save
the frame setup time. GDB has limited facilities for dealing
with these function invocations. If the innermost function invocation
has no stack frame, GDB nevertheless regards it as though
it had a separate frame, which is numbered zero as usual, allowing
correct tracing of the function call chain. However, GDB has
no provision for frameless functions elsewhere in the stack.
frame args
frame command allows you to move from one stack frame to another,
and to print the stack frame you select. args may be either the
address of the frame or the stack frame number. Without an argument,
frame prints the current stack frame.
select-frame
select-frame command allows you to move from one stack frame
to another without printing the frame. This is the silent version of
frame.
A backtrace is a summary of how your 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
backtrace n
bt n
backtrace -n
bt -n
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 does not begin with a program counter
value, indicating that your program has stopped at the beginning of the
code for line 993 of builtin.c.
Most commands for examining the stack and other data in your 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
main.
frame addr
f addr
frame needs two addresses to
select an arbitrary frame: a frame pointer and a stack pointer.
On the MIPS and Alpha architecture, it needs two addresses: a stack
pointer and a program counter.
On the 29k architecture, it needs three addresses: a register stack
pointer, a program counter, and a memory stack pointer.
up n
down n
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:
(gdb) 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
prints ten lines centered on the point of execution in the frame.
See section Printing source lines.
up-silently n
down-silently n
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 GDB command scripts, where the output might be unnecessary and
distracting.
There are several other commands to print information about the selected stack frame.
frame
f
f. With an
argument, this command is used to select a stack frame.
See section Selecting a frame.
info frame
info f
info frame addr
info f addr
frame command.
See section Selecting a frame.
info args
info locals
info catch
up,
down, or frame commands); then type info catch.
See section Breakpoints and exceptions.
MIPS based computers use an unusual stack frame, which sometimes requires GDB to search backward in the object code to find the beginning of a function.
To improve response time (especially for embedded applications, where GDB may be restricted to a slow serial line for this search) you may want to limit the size of this search, using one of these commands:
set heuristic-fence-post limit
heuristic-fence-post must search and
therefore the longer it takes to run.
show heuristic-fence-post
These commands are available only when GDB is configured for debugging programs on MIPS processors.
GDB can print parts of your program's source, since the debugging information recorded in the program tells GDB what source files were used to build it. When your program stops, GDB spontaneously prints the line where it stopped. Likewise, when you select a stack frame (see section Selecting a frame), GDB prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command.
If you use GDB through its GNU Emacs interface, you may prefer to use Emacs facilities to view source; see section Using GDB under GNU Emacs.
To print lines from a source file, use the list command
(abbreviated l). By default, ten lines are printed.
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
list function
list
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 -
By default, GDB prints ten source lines with any of these forms of
the list command. You can change this using set listsize:
set listsize count
list command display count source lines (unless
the list argument explicitly specifies some other number).
show listsize
list prints.
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
list first,last
list ,last
list first,
list +
list -
list
Here are the ways of specifying a single source line--all the kinds of linespec.
number
list command has two linespecs, this refers to
the same source file as the first linespec.
+offset
list command that has
two, this specifies the line offset lines down from the
first linespec.
-offset
filename:number
function
filename:function
*address
There are two commands for searching through the current source file for a regular expression.
forward-search regexp
search regexp
fo.
reverse-search regexp
rev.
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. GDB has a list of directories to search for source files; this is called the source path. Each time GDB 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 GDB cannot find a source file in the source path, and the object program records a directory, GDB tries that directory too. If the source path is empty, and there is no record of the compilation directory, GDB looks in the current directory as a last resort.
Whenever you reset or rearrange the source path, GDB clears out any information it has cached about where source files are found and where each line is in the file.
When you start GDB, its source path is empty.
To add other directories, use the directory command.
directory dirname ...
dir dirname ...
directory
show directories
If your source path is cluttered with directories that are no longer of interest, GDB may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows:
directory with no argument to reset the source path to empty.
directory with suitable arguments to reinstall the
directories you want in the source path. You can add all the
directories in one command.
You can use the command info line to map source lines to program
addresses (and vice versa), and the command disassemble to display
a range of addresses as machine instructions. When run under GNU Emacs
mode, the info line command now causes the arrow to point to the
line specified. Also, info line prints addresses in symbolic form as
well as hex.
info line linespec
list command (see section Printing source lines).
For example, we can use info line to discover the location of
the object code for the first line of function
m4_changequote:
(gdb) 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:
(gdb) 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
We can use disassemble to inspect the object code
range shown in the last info line example (the example
shows SPARC machine instructions):
(gdb) 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.
set assembly-language instruction-set
disassemble or x/i commands. It is useful for architectures that
have more than one native instruction set.
Currently it is only defined for the Intel x86 family. You can set instruction-set
to either i386 or i8086. The default is i386.
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 GDB with Different Languages).
print exp
print /f exp
print
print /f
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 are 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.
print and many other GDB commands accept an expression and
compute its value. Any kind of constant, variable or operator defined
by the programming language you are using is valid in an expression in
GDB. This includes conditional expressions, function calls, casts
and string constants. It unfortunately does not include symbols defined
by preprocessor #define commands.
GDB now supports array constants in expressions input by
the user. The syntax is {element, element...}. For example,
you can now use the command print {1, 2, 3} to build up an array in
memory that is malloc'd in the target program.
Because C is so widespread, most of the expressions shown in examples in this manual are in C. See section Using GDB with Different Languages, for information on how to use expressions in other languages.
In this section, we discuss operators that you can use in GDB 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 in order to examine a structure at that address in memory.
GDB supports these operators, in addition to those common to programming languages:
@
::
{type} addr
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 be either:
or
This means that in the function
foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}
}
you can examine and use the variable a whenever your program is
executing within the function foo, but you can only use or
examine the variable b while your program is executing inside
the block where 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 static variable in a particular function or file, using the colon-colon notation:
file::variable function::variable
Here file or function is the name of the context for the
static variable. In the case of file names, you can use quotes to
make sure GDB parses the file name as a single word--for example,
to print a global value of x defined in `f2.c':
(gdb) p 'f2.c'::x
This use of `::' is very rarely in conflict with the very similar use of the same notation in C++. GDB also supports use of the C++ scope resolution operator in GDB expressions.
Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function--just after entry to a new scope, and just before exit.
You may see this problem when you are 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 are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On 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.
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.
You can do this by referring to a contiguous span of memory as an artificial array, using the binary operator `@'. The left operand of `@' should be the first element of the desired array and be 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.
Another way to create an artificial array is to use a cast. This re-interprets a value as if it were an array. The value need not be in memory:
(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}
As a convenience, if you leave the array length out (as in `(type)[])value') gdb calculates the size to fill the value (as `sizeof(value)/sizeof(type)':
(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}
Sometimes the artificial array mechanism is not quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent--for example, if you are 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 are interested in the values of a field fv
in each structure. Here is an example of what you might type:
set $i = 0 p dtab[$i++]->fv RET RET ...
By default, GDB 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
d
u
o
t
a
(gdb) p/a 0x54320 $3 = 0x54320 <_initialize_vx+396>
c
f
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 GDB 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.
You can use the command x (for "examine") to examine memory in
any of several formats, independently of your program's data types.
x/nfu addr
x addr
x
x command to examine memory.
n, f, and u are all optional parameters that 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.
print,
`s' (null-terminated string), or `i' (machine instruction).
The default is `x' (hexadecimal) initially.
The default changes each time you use either x or print.
b
h
w
g
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.)
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 do not have to remember whether unit size or format comes first; either order works. The output specifications `4xw' and `4wx' mean exactly the same thing. (However, the count n must come first; `wx4' does 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 have 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, GDB 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.
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 GDB prints its value each time your 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
This display shows 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
display does not repeat if you press RET again after using it.
display/fmt exp
display/fmt addr
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...
undisplay does not repeat if you press RET after using it.
(Otherwise you would just get the error `No display number ...'.)
disable display dnums...
enable display dnums...
display
info display
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, GDB displays this argument while your program
continues to stop inside that function. When it stops elsewhere--where
there is no variable last_char---the display is disabled
automatically. The next time your program stops where last_char
is meaningful, you can enable the display expression once again.
GDB 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
on. For example, this is what a stack frame display looks like with
set print address on:
(gdb) f
#0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
at input.c:530
530 if (lquote != def_lquote)
set print address off
set print address off:
(gdb) set print addr off (gdb) f #0 set_quotes (lq="<<", rq=">>") at input.c:530 530 if (lquote != def_lquote)You can use `set print address off' to eliminate all machine dependent displays from the GDB interface. For example, with
print address off, you should get the same text for backtraces on
all machines--whether or not they involve pointer arguments.
show print address
When GDB prints a symbolic address, it normally prints the
closest earlier symbol plus an offset. If that symbol does not uniquely
identify the address (for example, it is a name whose scope is a single
source file), you may need to clarify. One way to do this is with
info line, for example `info line *0x4537'. Alternately,
you can set GDB to print the source file and line number when
it prints a symbolic address:
set print symbol-filename on
set print symbol-filename off
show print symbol-filename
Another situation where it is helpful to show symbol filenames and line numbers is when disassembling code; GDB shows you the line number and source file that corresponds to each instruction.
Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol:
set print max-symbolic-offset max-offset
show print max-symbolic-offset
If you have a pointer and you are not sure where it points, try
`set print symbol-filename on'. Then you can determine the name
and source file location of the variable where it points, using
`p/a pointer'. This interprets the address in symbolic form.
For example, here GDB shows that a variable ptt points
at another variable t, defined in `hi2.c':
(gdb) set print symbol-filename on (gdb) p/a ptt $4 = 0xe008 <t in hi2.c>
Warning: For pointers that point to a local variable, `p/a' does not show the symbol name and filename of the referent, even with the appropriate
set printoptions turned on.
Other settings control how different kinds of objects are printed:
set print array
set print array on
set print array off
show print array
set print elements number-of-elements
set print elements command.
This limit also applies to the display of strings.
Setting number-of-elements to zero means that the printing is unlimited.
show print elements
set print null-stop
set print pretty on
$1 = {
next = 0x0,
flags = {
sweet = 1,
sour = 1
},
meat = 0x54 "Pork"
}
set print pretty off
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}
This is the default format.
show print pretty
set print sevenbit-strings on
\nnn. This setting is
best if you are working in English (ASCII) and you use the
high-order bit of characters as a marker or "meta" bit.
set print sevenbit-strings off
show print sevenbit-strings
set print union on
set print union off
show print union
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
$1 = {it = Tree, form = {...}}
These settings are of interest when debugging C++ programs:
set print demangle
set print demangle on
show print demangle
set print asm-demangle
set print asm-demangle on
show print asm-demangle
set demangle-style style
auto
gnu
g++) encoding algorithm.
This is the default.
lucid
lcc) encoding algorithm.
arm
cfront-generated executables. GDB would
require further enhancement to permit that.
foo
show demangle-style
set print object
set print object on
set print object off
show print object
set print static-members
set print static-members on
set print static-members off
show print static-members
set print vtbl
set print vtbl on
set print vtbl off
show print vtbl
Values printed by the print command are saved in the GDB
value history. This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded
(for example with the file or symbol-file commands).
When the symbol table changes, the value history is discarded,
since the values may contain pointers back to the types defined in the
symbol table.
The values printed are given history numbers by which you can
refer to them. These are successive integers starting with one.
print shows you the history number assigned to a value by
printing `$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
show
values does not change the history.
show values n
show values +
show values + produces no display.
Pressing RET to repeat show values n has exactly the
same effect as `show values +'.
GDB provides convenience variables that you can use within GDB to hold on to a value and refer to it later. These variables exist entirely within GDB; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is 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. For 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
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:
set $i = 0 print bar[$i++]->contents
Repeat that command by typing RET.
Some convenience variables are created automatically by GDB and given values likely to be useful.
$_
$_ 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 $__.
$__
$__ 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.
$_exitcode
$_exitcode is automatically set to the exit code when
the program being debugged terminates.
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
info all-registers
info registers regname ...
GDB has four "standard" register names that are available (in
expressions) on most machines--whenever they do not 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(2) 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.
GDB 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, GDB 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, GDB must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if GDB is unable to locate the saved registers, the selected stack frame makes no difference.
set rstack_high_address address
set
rstack_high_address command. The argument should be an address, which
you probably want to precede with `0x' to specify in
hexadecimal.
show rstack_high_address
Depending on the configuration, GDB may be able to give you more information about the status of the floating point hardware.
info float
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 appear as
`0x1ae', while in Modula-2 they appear as `1AEH'.
Language-specific information is built into GDB for some languages, allowing you to express operations like the above in your program's native language, and allowing GDB to output values in a manner consistent with the syntax of your program's native language. The language you use to build expressions is called the working language.
There are two ways to control the working language--either have GDB
set it automatically, or select it manually yourself. You can use the
set language command for either purpose. On startup, GDB
defaults to setting the language automatically. The working language is
used to determine how expressions you type are interpreted, how values
are printed, etc.
In addition to the working language, every source file that
GDB knows about has its own working language. For some object
file formats, the compiler might indicate which language a particular
source file is in. However, most of the time GDB infers the
language from the name of the file. The language of a source file
controls whether C++ names are demangled--this way backtrace can
show each frame appropriately for its own language. There is no way to
set the language of a source file from within GDB.
This is most commonly a problem when you use a program, such
as cfront or f2c, that generates C but is written in
another language. In that case, make the
program use #line directives in its C output; that way
GDB will know the correct language of the source code of the original
program, and will display that source code, not the generated C code.
If a source file name ends in one of the following extensions, then GDB infers that its language is the one indicated.
If you allow GDB to set the language automatically, expressions are interpreted the same way in your debugging session and your program.
If you wish, you may set the language manually. To do this, issue the
command `set language lang', where lang is the name of
a language, such as
c or modula-2.
For a list of the supported languages, type `set language'.
Setting the language manually prevents GDB 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 GDB 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.
To have GDB set the working language automatically, use `set language local' or `set language auto'. GDB then infers the working language. That is, when your program stops in a frame (usually by encountering a breakpoint), GDB 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 GDB 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.
The following commands help you find out which language is the working language, and also what language source files were written in.
show language
print to
build and compute expressions that may involve variables in your program.
info frame
info source
Warning: In this release, the GDB 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 your program is running.
GDB can check for conditions like the above if you wish.
Although GDB does not check the statements in your program, it
can check expressions entered directly into GDB for evaluation via
the print command, for example. As with the working language,
GDB can also decide whether or not to check automatically based on
your program's source language. See section Supported languages,
for the default settings of supported languages.
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 but error--> 1 + 2.3
The second example fails because the CARDINAL 1 is not
type-compatible with the REAL 2.3.
For the expressions you use in GDB commands, you can tell the GDB type checker to skip checking; to treat any mismatches as errors and abandon the expression; or to only issue warnings when type mismatches occur, but evaluate the expression anyway. When you choose the last of these, GDB evaluates expressions like the second example above, but also issues a warning.
Even if you turn type checking off, there may be other reasons
related to type that prevent GDB from evaluating an expression.
For instance, GDB 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 further details on specific languages.
GDB provides some additional commands for controlling the type checker:
set check type auto
set check type on
set check type off
set check type warn
show type
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 GDB commands, you can tell GDB to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway.
A range error can result from numerical overflow, from exceeding an array index bound, or when you type 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.
GDB provides some additional commands for controlling the range checker:
set check range auto
set check range on
set check range off
set check range warn
show range
GDB 4 supports C, C++, and Modula-2.
Some GDB features may be used in expressions regardless of the
language you use: the GDB @ and :: operators,
and the `{type}addr' construct (see section Expressions) can be used with the constructs of any supported
language.
The following sections detail to what degree each source language is supported by GDB. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDB expression parser accepts, 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.
Since C and C++ are so closely related, many features of GDB apply to both languages. Whenever this is the case, we discuss those languages together.
The C++ debugging facilities are jointly implemented by the GNU C++
compiler and GDB. Therefore, to debug your C++ code
effectively, you must compile your C++ programs with the GNU C++
compiler, g++.
For best results when debugging C++ programs, use the stabs debugging
format. You can select that format explicitly with the g++
command-line options `-gstabs' or `-gstabs+'. See
section `Options for Debugging Your Program or GNU CC' in Using GNU CC, for more information.
Operators must be defined on values of specific types. For instance,
+ is defined on numbers, but not on structures. Operators are
often defined on groups of types.
For the purposes of C and C++, the following definitions hold:
int with any of its storage-class
specifiers; char; and enum.
float and double.
(type
*).
The following operators are supported. They are listed here in order of increasing precedence:
,
=
op=
a op= b,
and translated to a = a op b.
op= and = have the same precendence.
op is any one of the operators |, ^, &,
<<, >>, +, -, *, /, %.
?:
a ? b : c can be thought
of as: if a then b else c. a should be of an
integral type.
||
&&
|
^
&
==, !=
<, >, <=, >=
<<, >>
@
+, -
*, /, %
++, --
*
++.
&
++.
For debugging C++, GDB implements a use of `&' beyond what is
allowed in the C++ language itself: you can use `&(&ref)'
(or, if you prefer, simply `&&ref') to examine the address
where a C++ reference variable (declared with `&ref') is
stored.
-
++.
!
++.
~
++.
., ->
struct and union data.
[]
a[i] is defined as
*(a+i). Same precedence as ->.
()
->.
::
struct, union, and class types.
::
::, above.
GDB allows you to express the constants of C and C++ in the following ways:
long value.
'), or a number--the ordinal value of the corresponding character
(usually its ASCII value). Within quotes, the single character may
be represented by a letter or by escape sequences, which are of
the form `\nnn', where nnn is the octal representation
of the character's ordinal value; or of the form `\x', where
`x' is a predefined special character--for example,
`\n' for newline.
").
GDB expression handling has a number of extensions to interpret a significant subset of C++ expressions.
Warning: GDB can only debug C++ code if you compile with the GNU C++ compiler. Moreover, C++ debugging depends on the use of additional debugging information in the symbol table, and thus requires special support. GDB has this support only with the stabs debug format. In particular, if your compiler generates a.out, MIPS ECOFF, RS/6000 XCOFF, or ELF with stabs extensions to the symbol table, these facilities are all available. (With GNU CC, you can use the `-gstabs' option to request stabs debugging extensions explicitly.) Where the object code format is standard COFF or DWARF in ELF, on the other hand, most of the C++ support in GDB does not work.
count = aml->GetOriginal(x, y)
this following the same rules as C++.
::---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'. GDB also allows
resolving name scope by reference to source files, in both C and C++
debugging (see section Program variables).
If you allow GDB to set type and range checking automatically, they
both default to off whenever the working language changes to
C or C++. This happens regardless of whether you or GDB
selects the working language.
If you allow GDB to set the language automatically, it recognizes source files whose names end with `.c', `.C', or `.cc', and when GDB enters code compiled from one of these files, it sets the working language to C or C++. See section Having GDB infer the source language, for further details.
By default, when GDB parses C or C++ expressions, type checking is not used. However, if you turn type checking on, GDB considers two variables type equivalent if:
typedef.
Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array.
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
is also printed.
Otherwise, it appears as `{...}'.
The @ operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function. See section Expressions.
Some GDB commands are particularly useful with C++, and some are designed specifically for use with C++. Here is a summary:
breakpoint menus
rbreak regex
catch exceptions
info catch
ptype typename
set print demangle
show print demangle
set print asm-demangle
show print asm-demangle
set print object
show print object
set print vtbl
show print vtbl
Overloaded symbol names
symbol(types) rather than just symbol. You can
also use the GDB command-line word completion facilities to list the
available choices, or to finish the type list for you.
See section Command completion, for details on how to do this.
The extensions made to GDB to support Modula-2 only 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 is most likely to give an error as GDB reads in the executable's symbol table.
Operators must be defined on values of specific types. For instance,
+ is defined on numbers, but not on structures. Operators are
often defined on groups of types. For the purposes of Modula-2, the
following definitions hold:
INTEGER, CARDINAL, and
their subranges.
CHAR and its subranges.
REAL.
POINTER TO
type.
SET and BITSET types.
BOOLEAN.
The following operators are supported, and appear in order of increasing precedence:
,
:=
:= value is
value.
<, >
<=, >=
<.
=, <>, #
<. In GDB scripts, only <> is
available for inequality, since # conflicts with the script
comment character.
IN
<.
OR
AND, &
@
+, -
*
/
*.
DIV, MOD
*.
-
INTEGER and REAL data.
^
NOT
^.
.
RECORD field selector. Defined on RECORD data. Same
precedence as ^.
[]
ARRAY data. Same precedence as ^.
()
PROCEDURE objects. Same precedence
as ^.
::, .
Warning: Sets and their operations are not yet supported, so GDB treats the use of the operator
IN, or the use of operators+,-,*,/,=, ,<>,#,<=, and>=on sets as an error.
Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:
ARRAY variable.
CHAR constant or variable.
SET OF mtype (where mtype is the type of m).
All Modula-2 built-in procedures also return a result, described below.
ABS(n)
CAP(c)
CHR(i)
DEC(v)
DEC(v,i)
EXCL(m,s)
FLOAT(i)
HIGH(a)
INC(v)
INC(v,i)
INCL(m,s)
MAX(t)
MIN(t)
ODD(i)
ORD(x)
SIZE(x)
TRUNC(r)
VAL(t,i)
Warning: Sets and their operations are not yet supported, so GDB treats the use of procedures
INCLandEXCLas an error.
GDB allows you to express the constants of Modula-2 in the following ways:
') or double ("). They may
also be expressed by their ordinal value (their ASCII value, usually)
followed by a `C'.
') or double (").
Escape sequences in the style of C are also allowed. See section C and C++ constants, for a brief explanation of escape
sequences.
TRUE and
FALSE.
If type and range checking are set automatically by GDB, they
both default to on whenever the working language changes to
Modula-2. This happens regardless of whether you, or GDB,
selected the working language.
If you allow GDB to set the language automatically, then entering code compiled from a file whose name ends with `.mod' sets the working language to Modula-2. See section Having GDB infer the source language, for further details.
A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:
:=) returns the value of its right-hand
argument.
Warning: in this release, GDB does not yet perform type or range checking.
GDB considers two Modula-2 variables type equivalent if:
TYPE
t1 = t2 statement
As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error.
Range checking is done on all mathematical operations, assignment, array index bounds, and all built-in functions and procedures.
:: and .
There are a few subtle differences between the Modula-2 scope operator
(.) and the GDB 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 declared identifier within your program, except another module.
Using the :: operator makes GDB search the scope
specified by scope for the identifier id. If it is not
found in the specified scope, then GDB searches all scopes
enclosing the one specified by scope.
Using the . operator makes GDB 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.
Some GDB 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 the C 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)
In GDB scripts, the Modula-2 inequality operator # is
interpreted as the beginning of a comment. Use <> instead.
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 your program executes. GDB finds it in your program's symbol table, in the file indicated when you started GDB (see section Choosing files), or by one of the file-management commands (see section Commands to specify files).
Occasionally, you may need to refer to symbols that contain unusual characters, which GDB ordinarily treats as word delimiters. The most frequent case is in referring to static variables in other source files (see section Program variables). File names are recorded in object files as debugging symbols, but GDB would ordinarily parse a typical file name, like `foo.c', as the three words `foo' `.' `c'. To allow GDB to recognize `foo.c' as a single symbol, enclose it in single quotes; for example,
p 'foo.c'::x
looks up the value of x in the scope of the file `foo.c'.
info address symbol
whatis exp
whatis
$, the last value in the value history.
ptype typename
ptype exp
ptype
ptype
differs from whatis by printing a detailed description, instead
of just the name of the type.
For example, for this variable declaration:
struct complex {double real; double imag;} v;
the two commands give this output:
(gdb) whatis v
type = struct complex
(gdb) 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
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
info sources
info functions
info functions regexp
step; `info fun ^step' finds those whose names
start with step.
info variables
info variables regexp
set symbol-reloading on
set symbol-reloading off
symbol-reloading off, since otherwise GDB may discard symbols
when linking large programs, that may contain several modules (from
different directories or libraries) with the same name.
show symbol-reloading
on or off setting.
maint print symbols filename
maint print psymbols filename
maint print msymbols filename
info sources to find out which files these are. If you
use `maint print psymbols' instead, the dump shows information about
symbols that GDB only knows partially--that is, symbols defined in
files that GDB has skimmed, but not yet read completely. Finally,
`maint print msymbols' dumps just the minimal symbol information
required for each object file from which GDB has read some symbols.
See section Commands to specify files, for a discussion of how
GDB reads symbols (in the description of symbol-file).
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 alter the value of a variable, evaluate an assignment expression. See section 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).
See section Using GDB 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, 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
(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.
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
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
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 of where it
will run when you continue. For example,
set $pc = 0x485
makes the next continue command or stepping command execute at
address 0x485, rather than at the address where your 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.
signal signal
signal 2 and signal
SIGINT are both ways of sending an interrupt signal.
Alternatively, if signal 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.
Invoking the signal command is not the same as invoking the
kill utility from the shell. Sending a signal with kill
causes GDB to decide what to do with the signal depending on
the signal handling tables (see section Signals). The signal command
passes the signal directly to your program.
return
return expression
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 (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.
call expr
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. If the result is not void, it
is printed and saved in the value history.
A new user-controlled variable, call_scratch_address, specifies the location of a scratch area to be used when GDB calls a function in the target. This is necessary because the usual method of putting the scratch area on the stack does not work in systems that have separate instruction and data spaces.
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
exec-file
or core-file
command) after changing set write, for your new setting to take
effect.
show write
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.
You may want to specify executable and core dump file names. The usual way to do this is at start-up time, using the arguments to GDB's start-up commands (see section Getting In and Out of GDB).
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
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 file
`filename.syms' may hold symbol table information for
filename. If so, GDB maps in the symbol table from
`filename.syms', starting up more quickly. See the
descriptions of the file options `-mapped' and `-readnow'
(available on the command line, and with the commands file,
symbol-file, or add-symbol-file, described below),
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 ]
PATH
if necessary to locate your program. Omitting filename means to
discard information on the executable file.
symbol-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 does not repeat if you press RET again after
executing it once.
When GDB is configured for a particular environment, it
understands 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. See section Optional warnings and messages.)
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 ]
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 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 always attempts 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 ]
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
(see section Killing the child process).
load filename
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 ..."
The file is loaded at whatever address is specified in the executable.
For some object file formats, you can specify the load address when you
link the program; for other formats, like a.out, the object file format
specifies a fixed address.
On VxWorks, load links filename dynamically on the
current target system as well as adding its symbols in GDB.
With the Nindy interface to an Intel 960 board, load
downloads 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
(see section GDB and Hitachi microprocessors),
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 does not repeat if you press RET again after using it.
add-symbol-file filename address
add-symbol-file filename address [ -readnow ] [ -mapped ]
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.
You can specify address as an expression.
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 does 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.
add-shared-symbol-file
add-shared-symbol-file command can be used only under Harris' CXUX
operating system for the Motorola 88k. GDB automatically looks for
shared libraries, however if GDB does not find yours, you can run
add-shared-symbol-file. It takes no arguments.
section
section command changes the base address of section SECTION of
the exec file to ADDR. This can be used if the exec file does not contain
section addresses, (such as in the a.out format), or when the addresses
specified in the file itself are wrong. Each section must be changed
separately. The "info files" command lists all the sections and their
addresses.
info files
info target
info files and info target are synonymous; both print
the current target (see section Specifying a Debugging Target),
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 target 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 file name and remembers it that way.
GDB supports SunOS, SVr4, Irix 5, 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 does not understand
references to a function in a shared library, however--unless you are
debugging a core file).
info share
info sharedlibrary
sharedlibrary regex
share regex
run. If
regex is omitted all shared libraries required by your program are
loaded.
While reading a symbol file, GDB occasionally encounters 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 (see section Optional warnings and messages).
The messages currently printed, and their meanings, include:
inner block not inside outer block in symbol
(don't know)" if the outer block is not a
function.
block at address out of order
set verbose on. See section Optional warnings and messages.)
bad block start address patched
bad string table offset in symbol n
foo, which may cause other problems if many symbols end up
with this name.
unknown symbol type 0xnn
0xnn is the symbol type of the misunderstood
information, in hexadecimal.
GDB circumvents the error by ignoring this symbol information. This
usually allows you to debug your program, though certain symbols
are not 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
const/volatile indicator missing (ok if using g++ v1.x), got...
info mismatch between compiler and debugger
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 (see section Commands for managing 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 uses 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 (see section Commands to specify files). To specify as a target a process that is already running, use
the attach command (see section Debugging an already-running process).
target type parameters
target command does not repeat if you press RET again
after executing the command.
help target
info target or info files
(see section Commands to specify files).
help target name
set gnutarget args
set gnutarget command. Unlike most target commands,
with gnutarget the target refers to a program, not a machine.
Warning: To specify a file format with set gnutarget,
you must know the actual BFD name.
See section Commands to specify files.
show gnutarget
show gnutarget command to display what file format
gnutarget is set to read. If you have not set gnutarget,
GDB will determine the file format for each file automatically
and show gnutarget displays The current BDF target is "auto".
Here are some common targets (available, or not, depending on the GDB configuration):
target exec program
target core filename
target remote dev
target remote
now supports the load command. This is only useful if you have
some other way of getting the stub to the target system, and you can put
it somewhere in memory where it won't get clobbered by the download.
target sim
target udi keyword
target amd-eb dev speed PROG
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.
See section The EBMON protocol for AMD29K.
target hms dev
device and speed to control the serial
line and the communications speed used.
See section GDB and Hitachi microprocessors.
target nindy devicename
target st2000 dev speed
target vxworks machinename
target bug dev
target cpu32bug dev
target op50n dev
target w89k dev
target est dev
target rom68k dev
target array dev
target sparclite dev
Different targets are available on different configurations of GDB; your configuration may have more or fewer targets.
You can now choose which byte order to use with a target system.
Use the set endian big and set endian little commands.
Use the set endian auto command to instruct
GDB to use the byte order associated with the executable.
You can see the current setting for byte order with the show endian
command.
Warning: Currently, only embedded MIPS configurations support
dynamic selection of target byte order.
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 runs on the remote system to communicate with GDB.
Other remote targets may be available in your
configuration of GDB; use help target to list them.
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:
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:
gdbserver instead of linking a stub into your program.
See section Using the gdbserver program, for details.
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:
i386-stub.c
m68k-stub.c
sh-stub.c
sparc-stub.c
sparcl-stub.c
The `README' file in the GDB distribution may list other recently added stubs.
The debugging stub for your architecture supplies these three subroutines:
set_debug_traps
handle_exception to run when your
program stops. You must call this subroutine explicitly near the
beginning of your program.
handle_exception
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
handle_exception---in effect, 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 gets 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.
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.
First of all you need to tell the stub how to communicate with the serial port.
int getDebugChar()
getchar for your target system; a
different name is used to allow you to distinguish the two if you wish.
void putDebugChar(int)
putchar for your target system; a
different name is used to allow you to distinguish the two if you wish.
If you want GDB to be able to stop your program while it is
running, you need to use an interrupt-driven serial driver, and arrange
for it to stop when it receives a ^C (`\003', the control-C
character). That is the character which GDB uses to tell the
remote system to stop.
Getting the debugging target to return the proper status to GDB
probably requires changes to the standard stub; one quick and dirty way
is to just execute a breakpoint instruction (the "dirty" part is that
GDB reports a SIGTRAP instead of a SIGINT).
Other routines you need to supply are:
void exceptionHandler (int exception_number, void *exception_address)
exceptionHandler.
void flush_i_cache()
You must also make sure this library routine is available:
void *memset(void *, int, int)
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 varies 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.
In summary, when your program is ready to debug, you must follow these steps.
getDebugChar,putDebugChar,flush_i_cache,memset,exceptionHandler.
set_debug_traps(); breakpoint();
exceptionHook. Normally you just use:
void (*exceptionHook)() = 0;but if before calling
set_debug_traps, you set it to point to a
function in your program, that function is called when
GDB continues after stopping on a trap (for example, bus
error). The function indicated by exceptionHook is called with
one parameter: an int which is the exception number.
target remote command.
Its argument specifies how to communicate with the target
machine--either via a devicename attached to a direct serial line, or a
TCP port (usually to a terminal server which in turn has a serial line
to the target). For example, to use a serial line connected to the
device named `/dev/ttyb':
target remote /dev/ttybTo use a TCP connection, use an argument of the form
host:port. For example, to connect to port 2828 on a
terminal server named manyfarms:
target remote manyfarms:2828
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.
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 some of the commands currently supported (for a complete list of commands, look in `gdb/remote.c.'):
g
G
maddr,count
Maddr,count:...
c
caddr
s
saddr
k
?
T
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 shows you its current state.
gdbserver program
gdbserver is a control program for Unix-like systems, which
allows you to connect your program with a remote GDB via
target remote---but without linking in the usual debugging stub.
gdbserver is not a complete replacement for the debugging stubs,
because it requires essentially the same operating-system facilities
that GDB itself does. In fact, a system that can run
gdbserver to connect to a remote GDB could also run
GDB locally! gdbserver is sometimes useful nevertheless,
because it is a much smaller program than GDB itself. It is
also easier to port than all of GDB, so you may be able to get
started more quickly on a new system by using gdbserver.
Finally, if you develop code for real-time systems, you may find that
the tradeoffs involved in real-time operation make it more convenient to
do as much development work as possible on another system, for example
by cross-compiling. You can use gdbserver to make a similar
choice for debugging.
GDB and gdbserver communicate via either a serial line
or a TCP connection, using the standard GDB remote serial
protocol.
gdbserver does not need your program's symbol table, so you can
strip the program if necessary to save space. GDB on the host
system does all the symbol handling.
To use the server, you must tell it how to communicate with GDB;
the name of your program; and the arguments for your program. The
syntax is:
target> gdbserver comm program [ args ... ]comm is either a device name (to use a serial line) or a TCP hostname and portnumber. For example, to debug Emacs with the argument `foo.txt' and communicate with GDB over the serial port `/dev/com1':
target> gdbserver /dev/com1 emacs foo.txt
gdbserver waits passively for the host GDB to communicate
with it.
To use a TCP connection instead of a serial line:
target> gdbserver host:2345 emacs foo.txtThe only difference from the previous example is the first argument, specifying that you are communicating with the host GDB via TCP. The `host:2345' argument means that
gdbserver is to
expect a TCP connection from machine `host' to local TCP port 2345.
(Currently, the `host' part is ignored.) You can choose any number
you want for the port number as long as it does not conflict with any
TCP ports already in use on the target system (for example, 23 is
reserved for telnet).(3) You must use the same port number with the host GDB
target remote command.
target
remote to establish communications with gdbserver. Its argument
is either a device name (usually a serial device, like
`/dev/ttyb'), or a TCP port descriptor in the form
host:PORT. For example:
(gdb) target remote /dev/ttybcommunicates with the server via serial line `/dev/ttyb', and
(gdb) target remote the-target:2345communicates via a TCP connection to port 2345 on host `the-target'. For TCP connections, you must start up
gdbserver prior to using
the target remote command. Otherwise you may get an error whose
text depends on the host system, but which usually looks something like
`Connection refused'.
gdbserve.nlm program
gdbserve.nlm is a control program for NetWare systems, which
allows you to connect your program with a remote GDB via
target remote.
GDB and gdbserve.nlm communicate via a serial line,
using the standard GDB remote serial protocol.
gdbserve.nlm does not need your program's symbol table, so you
can strip the program if necessary to save space. GDB on the
host system does all the symbol handling.
To use the server, you must tell it how to communicate with
GDB; the name of your program; and the arguments for your
program. The syntax is:
load gdbserve [ BOARD=board ] [ PORT=port ]
[ BAUD=baud ] program [ args ... ]
board and port specify the serial line; baud specifies
the baud rate used by the connection. port and node default
to 0, baud defaults to 9600 bps.
For example, to debug Emacs with the argument `foo.txt'and
communicate with GDB over serial port number 2 or board 1
using a 19200 bps connection:
load gdbserve BOARD=1 PORT=2 BAUD=19200 emacs foo.txt
target
remote to establish communications with gdbserve.nlm. Its
argument is a device name (usually a serial device, like
`/dev/ttyb'). For example:
(gdb) target remote /dev/ttybcommunications with the server via serial line `/dev/ttyb'.
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:
target command at any point during your GDB
session. See section Commands for managing targets.
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 (see section Commands for managing targets).
These are the startup options for beginning your GDB session with a Nindy-960 board attached:
-r port
tty (e.g. `-r a').
-O
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
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.
reset
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
AMD distributes a 29K development board meant to fit in a PC, together
with a DOS-hosted monitor program called EBMON. As a shorthand
term, this development system is called the "EB29K". To use
GDB from a Unix system to run programs on the EB29K board, you
must first connect a serial cable between the PC (which hosts the EB29K
board) 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.
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 keeps
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 does not download it over the
serial line.
Finally, cd to the directory containing an image of your 29K
program on the Unix system, and start GDB---specifying as argument the
name of your 29K program:
cd /usr/joe/work29k gdb myfoo
Now you can use the target command:
target amd-eb /dev/ttya 9600 MYFOO
In this example, we've assumed your program is in a file called
`myfoo'. Note that the filename given as the last argument to
target amd-eb should be the name of the program as it appears to DOS.
In our example this is simply MYFOO, but in general it can include
a DOS path, and depending on your transfer mechanism may not resemble
the name on the Unix side.
At this point, you can set any breakpoints you wish; when you are ready
to see your program run on the 29K board, use the GDB command
run.
To stop debugging the remote program, use the GDB detach
command.
To return control of the PC to its console, use tip or cu
once again, after your GDB session has concluded, to attach to
EBMON. You can then type the command q to shut down
EBMON, returning control to the DOS command-line interpreter.
Type CTTY con to return command input to the main DOS console,
and type ~. to leave tip or cu.
The target amd-eb command creates a file `eb.log' in the
current working directory, to help debug problems with the connection.
`eb.log' records all the output from EBMON, including echoes
of the commands sent to it. Running `tail -f' on this file in
another window often helps to understand trouble with EBMON, or
unexpected events on the PC side of the connection.
To connect your ST2000 to the host system, see the manufacturer's manual. Once the ST2000 is physically attached, you can run:
target st2000 dev speed
to establish it as your debugging environment. dev is normally
the name of a serial device, such as `/dev/ttya', connected to the
ST2000 via a serial line. You can instead specify dev as a TCP
connection (for example, to a serial line attached via a terminal
concentrator) using the syntax hostname:portnumber.
The load and attach commands are not defined for
this target; you must load your program into the ST2000 as you normally
would for standalone operation. GDB reads debugging information
(such as symbols) from a separate, debugging version of the program
available on your host computer.
These auxiliary GDB commands are available to help you with the ST2000 environment:
st2000 command
connect
GDB enables developers to spawn and debug tasks running on networked
VxWorks targets from a Unix host. Already-running tasks spawned from
the VxWorks shell can also be debugged. GDB uses code that runs on
both the Unix host and on the VxWorks target. The program
gdb is installed and executed on the Unix host. (It may be
installed with the name vxgdb, to distinguish it from a
GDB for debugging programs on the host itself.)
VxWorks-timeout args
vxworks-timeout.
This option is set by the user, and args represents the number of
seconds GDB waits for responses to rpc's. You might use this if
your VxWorks target is a slow software simulator or is on the far side
of a thin network line.
The following information on connecting to VxWorks was current when this manual was produced; newer releases of VxWorks may use revised procedures.
To use GDB with VxWorks, you must rebuild your VxWorks kernel
to include the remote debugging interface routines in the VxWorks
library `rdb.a'. To do this, define INCLUDE_RDB in the
VxWorks configuration file `configAll.h' and rebuild your VxWorks
kernel. The resulting kernel contains `rdb.a', and spawns the
source debugging task tRdbTask when VxWorks is booted. For more
information on configuring and remaking VxWorks, see the manufacturer's
manual.
Once you have included `rdb.a' in your VxWorks system image and set
your Unix execution search path to find GDB, you are ready to
run GDB. From your Unix host, run gdb (or vxgdb,
depending on your installation).
GDB comes up showing the prompt:
(vxgdb)
The GDB command target lets you connect to a VxWorks target on the
network. To connect to a target whose host name is "tt", type:
(vxgdb) target vxworks tt
GDB displays messages like these:
Attaching remote machine across net... Connected to tt.
GDB then attempts to read the symbol tables of any object modules loaded into the VxWorks target since it was last booted. GDB locates these files by searching the directories listed in the command search path (see section Your program's environment); if it fails to find an object file, it displays a message such as:
prog.o: No such file or directory.
When this happens, add the appropriate directory to the search path with
the GDB command path, and execute the target
command again.
If you have connected to the VxWorks target and you want to debug an
object that has not yet been loaded, you can use the GDB
load command to download a file from Unix to VxWorks
incrementally. The object file given as an argument to the load
command is actually opened twice: first by the VxWorks target in order
to download the code, then by GDB in order to read the symbol
table. This can lead to problems if the current working directories on
the two systems differ. If both systems have NFS mounted the same
filesystems, you can avoid these problems by using absolute paths.
Otherwise, it is simplest to set the working directory on both systems
to the directory in which the object file resides, and then to reference
the file by its name, without any path. For instance, a program
`prog.o' may reside in `vxpath/vw/demo/rdb' in VxWorks
and in `hostpath/vw/demo/rdb' on the host. To load this
program, type this on VxWorks:
-> cd "vxpath/vw/demo/rdb"
v Then, in GDB, type:
(vxgdb) cd hostpath/vw/demo/rdb (vxgdb) load prog.o
GDB displays a response similar to this:
Reading symbol data from wherever/vw/demo/rdb/prog.o... done.
You can also use the load command to reload an object module
after editing and recompiling the corresponding source file. Note that
this makes GDB delete all currently-defined breakpoints,
auto-displays, and convenience variables, and to clear the value
history. (This is necessary in order to preserve the integrity of
debugger data structures that reference the target system's symbol
table.)
You can also attach to an existing task using the attach command as
follows:
(vxgdb) attach task
where task is the VxWorks hexadecimal task ID. The task can be running or suspended when you attach to it. Running tasks are suspended at the time of attachment.
GDB enables developers to debug tasks running on
Sparclet targets from a Unix host.
GDB uses code that runs on
both the Unix host and on the Sparclet target. The program
gdb is installed and executed on the Unix host.
timeout args
remotetimeout.
This option is set by the user, and args represents the number of
seconds GDB waits for responses.
When compiling for debugging, include the options "-g" to get debug information and "-Ttext" to relocate the program to where you wish to load it on the target. You may also want to add the options "-n" or "-N" in order to reduce the size of the sections.
sparclet-aout-gcc prog.c -Ttext 0x12010000 -g -o prog -N
You can use objdump to verify that the addresses are what you intended.
sparclet-aout-objdump --headers --syms prog
Once you have set
your Unix execution search path to find GDB, you are ready to
run GDB. From your Unix host, run gdb
(or sparclet-aout-gdb, depending on your installation).
GDB comes up showing the prompt:
(gdbslet)
The GDB command file lets you choose with program to debug.
(gdbslet) file prog
GDB then attempts to read the symbol table of `prog'. GDB locates the file by searching the directories listed in the command search path. If the file was compiled with debug information (option "-g"), source files will be searched as well. GDB locates the source files by searching the directories listed in the directory search path (see section Your program's environment). If it fails to find a file, it displays a message such as:
prog: No such file or directory.
When this happens, add the appropriate directories to the search paths with
the GDB commands path and dir, and execute the
target command again.
The GDB command target lets you connect to a Sparclet target.
To connect to a target on serial port "ttya", type:
(gdbslet) target sparclet /dev/ttya Remote target sparclet connected to /dev/ttya main () at ../prog.c:3
GDB displays messages like these:
Connected to ttya.
Once connected to the Sparclet target,
you can use the GDB
load command to download the file from the host to the target.
The file name and load offset should be given as arguments to the load
command.
Since the file format is aout, the program must be loaded to the starting
address. You can use objdump to find out what this value is. The load
offset is an offset which is added to the VMA (virtual memory address)
of each of the file's sections.
For instance, if the program
`prog' was linked to text address 0x1201000, with data at 0x12010160
and bss at 0x12010170, in GDB, type:
(gdbslet) load prog 0x12010000 Loading section .text, size 0xdb0 vma 0x12010000
If the code is loaded at a different address then what the program was linked
to, you may need to use the section and add-symbol-file commands
to tell GDB where to map the symbol table.
You can now begin debugging the task using GDB's execution control
commands, b, step, run, etc. See the GDB
manual for the list of commands.
(gdbslet) b main Breakpoint 1 at 0x12010000: file prog.c, line 3. (gdbslet) run Starting program: prog Breakpoint 1, main (argc=1, argv=0xeffff21c) at prog.c:3 3 char *symarg = 0; (gdbslet) step 4 char *execarg = "hello!"; (gdbslet)
GDB needs to know these things to talk to your Hitachi SH, H8/300, or H8/500:
Use the special gdb command `device port' if you
need to explicitly set the serial device. The default port is the
first available port on your host. This is only necessary on Unix
hosts, where it is typically something like `/dev/ttya'.
gdb has another special command to set the communications
speed: `speed bps'. This command also is only used from Unix
hosts; on DOS hosts, set the line speed as usual from outside GDB with
the DOS mode command (for instance, `mode
com2:9600,n,8,1,p' for a 9600 bps connection).
The `device' and `speed' commands are available only when you
use a Unix host to debug your Hitachi microprocessor programs. If you
use a DOS host,
GDB depends on an auxiliary terminate-and-stay-resident program
called asynctsr to communicate with the development board
through a PC serial port. You must also use the DOS mode command
to set up the serial port on the DOS side.
You can use the E7000 in-circuit emulator to develop code for either the Hitachi SH or the H8/300H. Use one of these forms of the `target e7000' command to connect GDB to your E7000:
target e7000 port speed
target e7000 hostname
telnet to connect.
Some GDB commands are available only on the H8/300 or the H8/500 configurations:
set machine h8300
set machine h8300h
set memory mod
show memory
small,
big, medium, and compact.
GDB can use the MIPS remote debugging protocol to talk to a MIPS board attached to a serial line. This is available when you configure GDB with `--target=mips-idt-ecoff'.
Use these GDB commands to specify the connection to your target board:
target mips port
gdb with the
name of your program as the argument. To connect to the board, use the
command `target mips port', where port is the name of
the serial port connected to the board. If the program has not already
been downloaded to the board, you may use the load command to
download it. You can then use all the usual GDB commands.
For example, this sequence connects to the target board through a serial
port, and loads and runs a program called prog through the
debugger:
host$ gdb prog GDB is free software and ... (gdb) target mips /dev/ttyb (gdb) load prog (gdb) run
target mips hostname:portnumber
target pmon port
target ddb port
target lsi port
GDB also supports these special commands for MIPS targets:
set processor args
show processor
set processor command to set the type of MIPS
processor when you want to access processor-type-specific registers.
For example, set processor r3041 tells GDB
to use the CPO registers appropriate for the 3041 chip.
Use the show processor command to see what MIPS processor GDB
is using. Use the info reg command to see what registers
GDB is using.
set mipsfpu double
set mipsfpu single
set mipsfpu none
show mipsfpu
mipsfpu variable with
`show mipsfpu'.
set remotedebug n
show remotedebug
remotedebug variable. If you set it to 1 using
`set remotedebug 1', every packet is displayed. If you set it
to 2, every character is displayed. You can check the current value
at any time with the command `show remotedebug'.
set timeout seconds
set retransmit-timeout seconds
show timeout
show retransmit-timeout
set timeout seconds command. The
default is 5 seconds. Similarly, you can control the timeout used while
waiting for an acknowledgement of a packet with the set
retransmit-timeout seconds command. The default is 3 seconds.
You can inspect both values with show timeout and show
retransmit-timeout. (These commands are only available when
GDB is configured for `--target=mips-idt-ecoff'.)
The timeout set by set timeout does not apply when GDB
is waiting for your program to stop. In that case, GDB waits
forever because it has no way of knowing how long the program is going
to run before stopping.
For some configurations, GDB includes a CPU simulator that you can use instead of a hardware CPU to debug your programs. Currently, a simulator is available when GDB is configured to debug Zilog Z8000 or Hitachi microprocessor targets.
For the Z8000 family, `target sim' simulates either the Z8002 (the unsegmented variant of the Z8000 architecture) or the Z8001 (the segmented variant). The simulator recognizes which architecture is appropriate by inspecting the object code.
target sim
After specifying this target, you can debug programs for the simulated
CPU in the same style as programs for your host computer; use the
file command to load a new program image, the run command
to run your program, and so on.
As well as making available all the usual machine registers (see
info reg), this debugging target provides three additional items
of information as specially named registers:
cycles
insts
time
You can refer to these values in GDB expressions with the usual conventions; for example, `b fputc if $cycles>5000' sets a conditional breakpoint that suspends only after at least 5000 simulated clock ticks.
You can alter the way GDB interacts with you by using
the set command. For commands controlling how GDB displays
data, see section Print settings; other settings are described
here.
GDB indicates its readiness to read a command by printing a string
called the prompt. This string is normally `(gdb)'. You
can change the prompt string with the set prompt command. For
instance, when debugging GDB with GDB, it is useful to change
the prompt in one of the GDB sessions so that you can always tell
which one you are talking to.
Note: set prompt no longer adds a space for you after the
prompt you set. This allows you to set a prompt which ends in a space
or a prompt that does not.
set prompt newprompt
show prompt
GDB reads its input commands via the readline interface. This
GNU library provides consistent behavior for programs which provide a
command line interface to the user. Advantages are GNU Emacs-style
or vi-style inline editing of commands, csh-like history
substitution, and a storage and recall of command history across
debugging sessions.
You may control the behavior of command line editing in GDB with the
command set.
set editing
set editing on
set editing off
show editing
GDB can keep track of the commands you type during your debugging sessions, so that you can be certain of precisely what happened. Use these commands to manage the GDB command history facility.
set history filename fname
GDBHISTFILE, or to
`./.gdb_history' if this variable is not set.
set history save
set history save on
set history filename command. By default, this option is disabled.
set history save off
set history size size
HISTSIZE, or to 256 if this variable is not set.
History expansion assigns special meaning to the character !.
Since ! is also the logical not operator in C, history expansion
is off by default. If you decide to enable history expansion with the
set history expansion on command, you may sometimes need to
follow ! (when it is used as logical not, in an expression) with
a space or a tab to prevent it from being expanded. The readline
history facilities do not attempt substitution on the strings
!= and !(, even when history expansion is enabled.
The commands to control history expansion are:
set history expansion on
set history expansion
set history expansion off
vi may wish to read it.
show history
show history filename
show history save
show history size
show history expansion
show history by itself displays all four states.
show commands
show commands n
show commands +
Certain commands to GDB may produce large amounts of information output to the screen. To help you read all of it, GDB pauses and asks you for input at the end of each page of output. Type RET when you want to continue the output, or q to discard the remaining output. Also, the screen width setting determines when to wrap lines of output. Depending on what is being printed, GDB tries to break the line at a readable place, rather than simply letting it overflow onto the following line.
Normally GDB knows the size of the screen from the termcap data base
together with the value of the TERM environment variable and the
stty rows and stty cols settings. If this is not correct,
you can override it with the set height and set
width commands:
set height lpp
show height
set width cpl
show width
set commands specify a screen height of lpp lines and
a screen width of cpl characters. The associated show
commands display the current settings.
If you specify a height of zero lines, GDB does not pause during
output no matter how long the output is. This is useful if output is to a
file or to an editor buffer.
Likewise, you can specify `set width 0' to prevent GDB
from wrapping its output.
You can always enter numbers in octal, decimal, or hexadecimal in GDB by
the usual conventions: octal numbers begin with `0', decimal
numbers end with `.', and hexadecimal numbers begin with `0x'.
Numbers that begin with none of these are, by default, entered in base
10; likewise, the default display for numbers--when no particular
format is specified--is base 10. You can change the default base for
both input and output with the set radix command.
set input-radix base
set radix 012 set radix 10. set radix 0xasets the base to decimal. On the other hand, `set radix 10' leaves the radix unchanged no matter what it was.
set output-radix base
show input-radix
show output-radix
By default, GDB is silent about its inner workings. If you are running
on a slow machine, you may want to use the set verbose command.
This makes GDB tell you when it does a lengthy internal operation, so
you will not think it has crashed.
Currently, the messages controlled by set verbose are those
which announce that the symbol table for a source file is being read;
see symbol-file in section Commands to specify files.
set verbose on
set verbose off
show verbose
set verbose is on or off.
By default, if GDB encounters bugs in the symbol table of an object file, it is silent; but if you are debugging a compiler, you may find this information useful (see section Errors reading symbol files).
set complaints limit
show complaints
By default, GDB is cautious, and asks what sometimes seems to be a lot of stupid questions to confirm certain commands. For example, if you try to run a program which is already running:
(gdb) run The program being debugged has been started already. Start it from the beginning? (y or n)
If you are willing to unflinchingly face the consequences of your own commands, you can disable this "feature":
set confirm off
set confirm on
show confirm
Aside from breakpoint commands (see section Breakpoint command lists), GDB provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files.
A user-defined command is a sequence of GDB commands to which
you assign a new name as a command. This is done with the define
command. User commands may accept up to 10 arguments separated by whitespace.
Arguments are accessed within the user command via $arg0...$arg9.
A trivial example:
define adder print $arg0 + $arg1 + $arg2
To execute the command use:
adder 1 2 3
This defines the command adder, which prints the sum of
its three arguments. Note the arguments are text substitutions, so they may
reference variables, use complex expressions, or even perform inferior
functions calls.
define commandname
define command. The end of these
commands is marked by a line containing end.
if
else, followed
by a series of commands that are only executed if the expression
was false. The end of the list is marked by a line containing end.
while
if: the command takes a single argument,
which is an expression to evaluate, and must be followed by the commands to
execute, one per line, terminated by an end.
The commands are executed repeatedly as long as the expression
evaluates to true.
document commandname
help. The command commandname must already be
defined. This command reads lines of documentation just as define
reads the lines of the command definition, ending with end.
After the document command is finished, help on command
commandname displays the documentation you have written.
You may use the document command again to change the
documentation of a command. Redefining the command with define
does not change the documentation.
help user-defined
show user
show user commandname
When user-defined commands are executed, the commands of the definition are not printed. An error in any command stops execution of the user-defined command.
If used interactively, commands that would ask for confirmation proceed without asking when used inside a user-defined command. Many GDB commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.
You may define hooks, which are a special kind of user-defined command. Whenever you run the command `foo', if the user-defined command `hook-foo' exists, it is executed (with no arguments) before that command.
In addition, a pseudo-command, `stop' exists. Defining (`hook-stop') makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed.
For example, to ignore SIGALRM signals while
single-stepping, but treat them normally during normal execution,
you could define:
define hook-stop handle SIGALRM nopass end define hook-run handle SIGALRM pass end define hook-continue handle SIGLARM pass end
You can define a hook for any single-word command in GDB, but
not for command aliases; you should define a hook for the basic command
name, e.g. backtrace rather than bt.
If an error occurs during the execution of your hook, execution of
GDB commands stops and GDB issues a prompt
(before the command that you actually typed had a chance to run).
If you try to define a hook which does not match any known command, you
get a warning from the define command.
A command file for GDB is a file of lines that are GDB commands. Comments (lines starting with #) may also be included. An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal.
When you start GDB, it automatically executes commands from its
init files. These are files named `.gdbinit'.
GDB reads the init file (if any) in your home directory, then
processes command line options and operands, and then reads the init
file (if any) in the current working directory. This is so the init
file in your home directory can set options (such as set
complaints) which affect the processing of the command line options and
operands. The init files are not executed if you use the `-nx'
option; see section Choosing modes.
On some configurations of GDB, the init file is known by a different name (these are typically environments where a specialized form of GDB may need to coexist with other forms, hence a different name for the specialized version's init file). These are the environments with special init file names:
You can also request the execution of a command file with the
source command:
source filename
The lines in a command file are executed sequentially. They are not printed as they are executed. An error in any command terminates execution of the command file.
Commands that would ask for confirmation if used interactively proceed without asking when used in a command file. Many GDB commands that normally print messages to say what they are doing omit the messages when called from command files.
During the execution of a command file or a user-defined command, normal GDB output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition. This section describes three commands useful for generating exactly the output you want.
echo text
echo This is some text\n\ which is continued\n\ onto several lines.\nproduces the same output as
echo This is some text\n echo which is continued\n echo onto several lines.\n
output expression
output/fmt expression
print. See section Output formats, for more information.
printf string, expressions...
printf (string, expressions...);For example, you can print two values in hex like this:
printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-fooThe only backslash-escape sequences that you can use in the format string are the simple ones that consist of backslash followed by a letter.
A special interface allows you to use GNU Emacs to view (and edit) the source files for the program you are debugging with GDB.
To use this interface, use the command M-x gdb in Emacs. Give the executable file you want to debug as an argument. This command starts GDB as a subprocess of Emacs, with input and output through a newly created Emacs buffer.
Using GDB under Emacs is just like using GDB normally except for two things:
This applies both to GDB commands and their output, and to the input and output done by the program you are debugging.
This is useful because it means that you can copy the text of previous commands and input them again; you can even use parts of the output in this way.
All the facilities of Emacs' Shell mode are available for interacting with your program. In particular, you can send signals the usual way--for example, C-c C-c for an interrupt, C-c C-z for a stop.
Each time GDB displays a stack frame, Emacs automatically finds the source file for that frame and puts an arrow (`=>') at the left margin of the current line. Emacs uses a separate buffer for source display, and splits the screen to show both your GDB session and the source.
Explicit GDB list or search commands still produce output as
usual, but you probably have no reason to use them from Emacs.
Warning: If the directory where your program resides is not your current directory, it can be easy to confuse Emacs about the location of the source files, in which case the auxiliary display buffer does not appear to show your source. GDB can find programs by searching your environment's
PATHvariable, so the GDB input and output session proceeds normally; but Emacs does not get enough information back from GDB to locate the source files in this situation. To avoid this problem, either start GDB mode from the directory where your program resides, or specify an absolute file name when prompted for the M-x gdb argument.A similar confusion can result if you use the GDB
filecommand to switch to debugging a program in some other location, from an existing GDB buffer in Emacs.
By default, M-x gdb calls the program called `gdb'. If
you need to call GDB by a different name (for example, if you keep
several configurations around, with different names) you can set the
Emacs variable gdb-command-name; for example,
(setq gdb-command-name "mygdb")
(preceded by ESC ESC, or typed in the *scratch* buffer, or
in your `.emacs' file) makes Emacs call the program named
"mygdb" instead.
In the GDB I/O buffer, you can use these special Emacs commands in addition to the standard Shell mode commands:
step command; also
update the display window to show the current file and location.
next command. Then update the display window
to show the current file and location.
stepi command; update
display window accordingly.
nexti command; update
display window accordingly.
finish command.
continue
command.
Warning: In Emacs v19, this command is C-c C-p.
up command.
Warning: In Emacs v19, this command is C-c C-u.
down command.
Warning: In Emacs v19, this command is C-c C-d.
disassemble by typing C-x &.
You can customize this further by defining elements of the list
gdb-print-command; once it is defined, you can format or
otherwise process numbers picked up by C-x & before they are
inserted. A numeric argument to C-x & indicates that you
wish special formatting, and also acts as an index to pick an element of the
list. If the list element is a string, the number to be inserted is
formatted using the Emacs function format; otherwise the number
is passed as an argument to the corresponding list element.
In any source file, the Emacs command C-x SPC (gdb-break)
tells GDB to set a breakpoint on the source line point is on.
If you accidentally delete the source-display buffer, an easy way to get
it back is to type the command f in the GDB buffer, to
request a frame display; when you run under Emacs, this recreates
the source buffer if necessary to show you the context of the current
frame.
The source files displayed in Emacs are in ordinary Emacs buffers which are visiting the source files in the usual way. You can edit the files with these buffers if you wish; but keep in mind that GDB communicates with Emacs in terms of line numbers. If you add or delete lines from the text, the line numbers that GDB knows cease to correspond properly with the code.
Your bug reports play an essential role in making GDB reliable.
Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case the principal function of a bug report is to help the entire community by making the next version of GDB work better. Bug reports are your contribution to the maintenance of GDB.
In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.
If you are not sure whether you have found a bug, here are some guidelines:
A number of companies and individuals offer support for GNU products. If you obtained GDB from a support organization, we recommend you contact that organization first.
You can find contact information for many support companies and individuals in the file `etc/SERVICE' in the GNU Emacs distribution.
In any event, we also recommend that you send bug reports for GDB to one of these addresses:
bug-gdb@prep.ai.mit.edu
{ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gdb
Do not send bug reports to `info-gdb', or to `help-gdb', or to any newsgroups. Most users of GDB do not want to receive bug reports. Those that do have arranged to receive `bug-gdb'.
The mailing list `bug-gdb' has a newsgroup `gnu.gdb.bug' which serves as a repeater. The mailing list and the newsgroup carry exactly the same messages. Often people think of posting bug reports to the newsgroup instead of mailing them. This appears to work, but it has one problem which can be crucial: a newsgroup posting often lacks a mail path back to the sender. Thus, if we need to ask for more information, we may be unable to reach you. For this reason, it is better to send bug reports to the mailing list.
As a last resort, send bug reports on paper to:
GNU Debugger Bugs Free Software Foundation Inc. 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA
The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and assume that some details do not matter. Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it does not, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the debugger into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us. Therefore, always write your bug reports on the assumption that the bug has not been reported previously.
Sometimes people give a few sketchy facts and ask, "Does this ring a bell?" Those bug reports are useless, and we urge everyone to refuse to respond to them except to chide the sender to report bugs properly.
To enable us to fix the bug, you should include all these things:
show version.
Without this, we will not know whether there is any point in looking for
the bug in the current version of GDB.
Here are some things that are not necessary:
The GDB 4 release includes an already-formatted reference card, ready for printing with PostScript or Ghostscript, in the `gdb' subdirectory of the main source directory(4). If you can use PostScript or Ghostscript with your printer, you can print the reference card immediately with `refcard.ps'.
The release also includes the source for the reference card. You can format it, using TeX, by typing:
make refcard.dvi
The GDB reference card is designed to print in landscape mode on US "letter" size paper; that is, on a sheet 11 inches wide by 8.5 inches high. You will need to specify this form of printing as an option to your DVI output program.
All the documentation for GDB comes as part of the machine-readable
distribution. The documentation is written in Texinfo format, which is
a documentation system that uses a single source file to produce both
on-line information and a printed manual. You can use one of the Info
formatting commands to create the on-line version of the documentation
and TeX (or texi2roff) to typeset the printed version.
GDB includes an already formatted copy of the on-line Info version of
this manual in the `gdb' subdirectory. The main Info file is
`gdb-version-number/gdb/gdb.info', and it refers to
subordinate files matching `gdb.info*' in the same directory. If
necessary, you can print out these files, or read them with any editor;
but they are easier to read using the info subsystem in GNU Emacs
or the standalone info program, available as part of the GNU
Texinfo distribution.
If you want to format these Info files yourself, you need one of the
Info formatting programs, such as texinfo-format-buffer or
makeinfo.
If you have makeinfo installed, and are in the top level GDB
source directory (`gdb-', in the case of version ), you can
make the Info file by typing:
cd gdb make gdb.info
If you want to typeset and print copies of this manual, you need TeX, a program to print its DVI output files, and `texinfo.tex', the Texinfo definitions file.
TeX is a typesetting program; it does not print files directly, but produces output files called DVI files. To print a typeset document, you need a program to print DVI files. If your system has TeX installed, chances are it has such a program. The precise command to use depends on your system; lpr -d is common; another (for PostScript devices) is dvips. The DVI print command may require a file name without any extension or a `.dvi' extension.
TeX also requires a macro definitions file called `texinfo.tex'. This file tells TeX how to typeset a document written in Texinfo format. On its own, TeX cannot either read or typeset a Texinfo file. `texinfo.tex' is distributed with GDB and is located in the `gdb-version-number/texinfo' directory.
If you have TeX and a DVI printer program installed, you can typeset and print this manual. First switch to the the `gdb' subdirectory of the main source directory (for example, to `gdb-/gdb') and then type:
make gdb.dvi
GDB comes with a configure script that automates the process
of preparing GDB for installation; you can then use make to
build the gdb program.
(5)
The GDB distribution includes all the source code you need for GDB in a single directory, whose name is usually composed by appending the version number to `gdb'.
For example, the GDB version distribution is in the `gdb-' directory. That directory contains:
gdb-/configure (and supporting files)
gdb-/gdb
gdb-/bfd
gdb-/include
gdb-/libiberty
gdb-/opcodes
gdb-/readline
gdb-/glob
gdb-/mmalloc
The simplest way to configure and build GDB is to run configure
from the `gdb-version-number' source directory, which in
this example is the `gdb-' directory.
First switch to the `gdb-version-number' source directory
if you are not already in it; then run configure. Pass the
identifier for the platform on which GDB will run as an
argument.
For example:
cd gdb- ./configure host make
where host is an identifier such as `sun4' or
`decstation', that identifies the platform where GDB will run.
(You can often leave off host; configure tries to guess the
correct value by examining your system.)
Running `configure host' and then running make builds the
`bfd', `readline', `mmalloc', and `libiberty'
libraries, then gdb itself. The configured source files, and the
binaries, are left in the corresponding source directories.
configure is a Bourne-shell (/bin/sh) script; if your
system does not recognize this automatically when you run a different
shell, you may need to run sh on it explicitly:
sh configure host
If you run configure from a directory that contains source
directories for multiple libraries or programs, such as the
`gdb-' source directory for version , configure
creates configuration files for every directory level underneath (unless
you tell it not to, with the `--norecursion' option).
You can run the configure script from any of the
subordinate directories in the GDB distribution if you only want to
configure that subdirectory, but be sure to specify a path to it.
For example, with version , type the following to configure only
the bfd subdirectory:
cd gdb-/bfd ../configure host
You can install gdb anywhere; it has no hardwired paths.
However, you should make sure that the shell on your path (named by
the `SHELL' environment variable) is publicly readable. Remember
that GDB uses the shell to start your program--some systems refuse to
let GDB debug child processes whose programs are not readable.
If you want to run GDB versions for several host or target machines,
you need a different gdb compiled for each combination of
host and target. configure is designed to make this easy by
allowing you to generate each configuration in a separate subdirectory,
rather than in the source directory. If your make program
handles the `VPATH' feature (GNU make does), running
make in each of these directories builds the gdb
program specified there.
To build gdb in a separate directory, run configure
with the `--srcdir' option to specify where to find the source.
(You also need to specify a path to find configure
itself from your working directory. If the path to configure
would be the same as the argument to `--srcdir', you can leave out
the `--srcdir' option; it is assumed.)
For example, with version , you can build GDB in a separate directory for a Sun 4 like this:
cd gdb- mkdir ../gdb-sun4 cd ../gdb-sun4 ../gdb-/configure sun4 make
When configure builds a configuration using a remote source
directory, it creates a tree for the binaries with the same structure
(and using the same names) as the tree under the source directory. In
the example, you'd find the Sun 4 library `libiberty.a' in the
directory `gdb-sun4/libiberty', and GDB itself in
`gdb-sun4/gdb'.
One popular reason to build several GDB configurations in separate
directories is to configure GDB for cross-compiling (where
GDB runs on one machine--the host---while debugging
programs that run on another machine--the target).
You specify a cross-debugging target by
giving the `--target=target' option to configure.
When you run make to build a program or library, you must run
it in a configured directory--whatever directory you were in when you
called configure (or one of its subdirectories).
The Makefile that configure generates in each source
directory also runs recursively. If you type make in a source
directory such as `gdb-' (or in a separate configured
directory configured with `--srcdir=dirname/gdb-'), you
will build all the required libraries, and then build GDB.
When you have multiple hosts or targets configured in separate
directories, you can run make on them in parallel (for example,
if they are NFS-mounted on each of the hosts); they will not interfere
with each other.
The specifications used for hosts and targets in the configure
script are based on a three-part naming scheme, but some short predefined
aliases are also supported. The full naming scheme encodes three pieces
of information in the following pattern:
architecture-vendor-os
For example, you can use the alias sun4 as a host argument,
or as the value for target in a --target=target
option. The equivalent full name is `sparc-sun-sunos4'.
The configure script accompanying GDB does not provide
any query facility to list all supported host and target names or
aliases. configure calls the Bourne shell script
config.sub to map abbreviations to full names; you can read the
script, if you wish, or you can use it to test your guesses on
abbreviations--for example:
% sh config.sub sun4 sparc-sun-sunos4.1.1 % sh config.sub sun3 m68k-sun-sunos4.1.1 % sh config.sub decstation mips-dec-ultrix4.2 % sh config.sub hp300bsd m68k-hp-bsd % sh config.sub i386v i386-unknown-sysv % sh config.sub i786v Invalid configuration `i786v': machine `i786v' not recognized
config.sub is also distributed in the GDB source
directory (`gdb-', for version ).
configure options
Here is a summary of the configure options and arguments that
are most often useful for building GDB. configure also has
several other options not listed here. See Info file `configure.info', node `What Configure Does', for a full explanation of configure.
configure [--help]
[--prefix=dir]
[--srcdir=dirname]
[--norecursion] [--rm]
[--target=target] host
You may introduce options with a single `-' rather than `--' if you prefer; but you may abbreviate option names if you use `--'.
--help
configure.
-prefix=dir
--srcdir=dirname
make, or another
make that implements the VPATH feature.configure writes configuration specific files in
the current directory, but arranges for them to use the source in the
directory dirname. configure creates directories under
the working directory in parallel to the source directories below
dirname.
--norecursion
configure is executed; do not
propagate configuration to subdirectories.
--rm
--target=target
host ...
configure accepts other options, for compatibility with
configuring other GNU tools recursively; but these are the only
options that affect GDB or its supporting libraries.
Jump to: # - $ - . - / - : - @ - a - b - c - d - e - f - g - h - i - j - k - l - m - n - o - p - q - r - s - t - u - v - w - x - z - {
# in Modula-2
$
$$
$_ and info breakpoints
$_ and info line
$_, $__, and value history
breakpoint subroutine, remote
heuristic-fence-post (MIPS)
remotedebug protocol
remotedebug, MIPS protocol
retransmit-timeout, MIPS protocol
target remote
target remote
timeout, MIPS protocol
`b' cannot be used because these format letters are also
used with the x command, where `b' stands for "byte";
see section Examining memory.
This is a way of removing
one word from the stack, on machines where stacks grow downward in
memory (most machines, nowadays). This assumes that the innermost
stack frame is selected; setting $sp is not allowed when other
stack frames are selected. To pop entire frames off the stack,
regardless of machine architecture, use return;
see section Returning from a function.
If you choose a port number that
conflicts with another service, gdbserver prints an error message
and exits.
In `gdb-/gdb/refcard.ps' of the version release.
If you have a more recent version of GDB than , look at the `README' file in the sources; we may have improved the installation procedures since publishing this manual.
This document was generated on 30 May 1999 using the texi2html translator version 1.52.