gdb(1)
CLI
gdb [opts] [prg [-c coredump | -p pid]]
gdb [opts] --args prg <prg-args>
opts:
-p <pid> attach to pid
-c <coredump> use <coredump>
-x <file> execute script <file> before prompt
-ex <cmd> execute command <cmd> before prompt
--tty <tty> set I/O tty for debugee
--batch run in batch mode, exit after processing options (eg used
for scripting)
Interactive usage
Misc
tty <tty>
Set <tty> as tty for debugee.
Make sure nobody reads from target tty, easiest is to spawn a shell
and run following in target tty:
> while true; do sleep 1024; done
sharedlibrary [<regex>]
Load symbols of shared libs loaded by debugee. Optionally use <regex>
to filter libs for symbol loading.
display [/FMT] <expr>
Print <expr> every time debugee stops. Eg print next instr, see
examples below.
undisplay [<num>]
Delete display expressions either all or one referenced by <num>.
info display
List display expressions.
Breakpoints
break [-qualified] <sym> thread <tnum>
Set a breakpoint only for a specific thread.
-qualified: Treat <sym> as fully qualified symbol (quiet handy to set
breakpoints on C symbols in C++ contexts)
break <sym> if <cond>
Set conditional breakpoint (see examples below).
delete [<num>]
Delete breakpoint either all or one referenced by <num>.
info break
List breakpoints.
cond <bp> <cond>
Make existing breakpoint <bp> conditional with <cond>.
cond <bp>
Remove condition from breakpoint <bp>.
tbreak
Set temporary breakpoint, will be deleted when hit.
Same syntax as `break`.
rbreak <regex>
Set breakpoints matching <regex>, where matching internally is done
on: .*<regex>.*
command [<bp_list>]
Define commands to run after breakpoint hit. If <bp_list> is not
specified attach command to last created breakpoint. Command block
terminated with 'end' token.
<bp_list>: Space separates list, eg 'command 2 5-8' to run command
for breakpoints: 2,5,6,7,8.
save break <file>
Save breakpoints to <file>. Can be loaded with the `source` command.
Watchpoints
watch [-location|-l] <expr> [thread <tnum>]
Create a watchpoint for <expr>, will break if <expr> is written to.
Watchpoints respect scope of variables, -l can be used to watch the
memory location instead.
rwatch ...
Sets a read watchpoint, will break if <expr> is read from.
awatch ...
Sets an access watchpoint, will break if <expr> is written to or read
from.
Catchpoints
catch load [<regex>]
Stop when shared libraries are loaded, optionally specify a <regex>
to stop only on matches.
catch unload [<regex>]
Stop when shared libraries are unloaded, optionally specify a <regex>
to stop only on matches.
catch throw
Stop when an exception is thrown.
catch rethrow
Stop when an exception is rethrown.
catch catch
Stop when an exception is caught.
catch fork
Stop at calls to fork (also stops at clones, as some systems
implement fork via clone).
catch syscall [<syscall> <syscall> ..]
Stop at syscall. If no argument is given, stop at all syscalls.
Optionally give a list of syscalls to stop at.
Inspection
info functions [<regex>]
List functions matching <regex>. List all functions if no <regex>
provided.
info variables [<regex>]
List variables matching <regex>. List all variables if no <regex>
provided.
info register [<reg> <reg> ..]
Dump content of all registers or only the specified <reg>ister.
Signal handling
info handle [<signal>]
Print how to handle <signal>. If no <signal> specified print for all
signals.
handle <signal> <action>
Configure how gdb handles <signal> sent to debugee.
<action>:
stop/nostop Catch signal in gdb and break.
print/noprint Print message when gdb catches signal.
pass/nopass Pass signal down to debugee.
catch signal <signal>
Create a catchpoint for <signal>.
Multi-threading
info thread
List all threads.
thread apply <id> [<id>] <command>
Run command on all threads listed by <id> (space separated list).
When 'all' is specified as <id> the <command> is run on all threads.
thread name <name>
The <name> for the current thread.
Multi-process
set follow-fork-mode <child | parent>
Specify which process to follow when debuggee makes a fork(2)
syscall.
set detach-on-fork <on | off>
Turn on/off detaching from new child processes (on by default).
Turning this off allows to debug multiple processes (inferiors) with
one gdb session.
info inferiors
List all processes gdb debugs.
inferior <id>
Switch to inferior with <id>.
Shell commands
shell <shell_cmd>
Run the shell_cmd and print the output, can also contain a pipeline.
pipe <gdb_cmd> | <shell_cmd>
Evaluate the gdb_cmd and run the shell_cmd which receives the output
of the gdb_cmd via stdin.
Source file locations
dir <path>
Add <path> to the beginning of the searh path for source files.
show dir
Show current search path.
set substitute-path <from> <to>
Add substitution rule checked during source file lookup.
show substitute-path
Show current substitution rules.
Configuration
set disassembly-flavor <intel | att>
Set the disassembly style "flavor".
set pagination <on | off>
Turn on/off gdb's pagination.
set breakpoint pending <on | off | auto>
on: always set pending breakpoints.
off: error when trying to set pending breakpoints.
auto: interatively query user to set breakpoint.
set print pretty <on | off>
Turn on/off pertty printing of structures.
set style enabled <on | off>
Turn on/off styling (eg colored output).
set logging <on | off>
Enable output logging to file (default gdb.txt).
set logging file <fname>
Change output log file to <fname>
set logging redirect <on/off>
on: only log to file.
off: log to file and tty.
set logging overwrite <on/off>
on: Truncate log file on each run.
off: Append to logfile (default).
Logging options should be configured before logging is turned on.
Text user interface (TUI)
C-x a Toggle UI.
C-l Redraw UI (curses UI can be messed up after the debugee prints to
stdout/stderr).
C-x o Change focus.
User commands (macros)
Gdb allows to create & document user commands as follows:
define <cmd>
# cmds
end
document <cmd>
# docu
end
To get all user commands or documentations one can use:
help user-defined
help <cmd>
Hooks
Gdb allows to create two types of command hooks
hook-will be run before<cmd>hookpost-will be run after<cmd>
define hook-<cmd>
# cmds
end
define hookpost-<cmd>
# cmds
end
Examples
Automatically print next instr
When ever the debugee stops automatically print the memory at the current
instruction pointer ($rip x86) and format as instruction /i.
# rip - x86
display /i $rip
# step instruction, after the step the next instruction is automatically printed
si
Conditional breakpoints
Create conditional breakpoints for a function void foo(int i) in the debugee.
# Create conditional breakpoint
b foo if i == 42
b foo # would create bp 2
# Make existing breakpoint conditional
cond 2 i == 7
Set breakpoint on all threads except one
Create conditional breakpoint using the $_thread convenience
variable.
# Create conditional breakpoint on all threads except thread 12.
b foo if $_thread != 12
Catch SIGSEGV and execute commands
This creates a catchpoint for the SIGSEGV signal and attached the command
to it.
catch signal SIGSEGV
command
bt
c
end
Run backtrace on thread 1 (batch mode)
gdb --batch -ex 'thread 1' -ex 'bt' -p <pid>
Script gdb for automating debugging sessions
To script gdb add commands into a file and pass it to gdb via -x.
For example create run.gdb:
set pagination off
break mmap
command
info reg rdi rsi rdx
bt
c
end
#initial drop
c
This script can be used as:
gdb --batch -x ./run.gdb -p <pid>
Hook to automatically save breakpoints on quit
define break-save
save breakpoint $arg0.gdb.bp
end
define break-load
source $arg0.gdb.bp
end
define hook-quit
break-save quit
end
Watchpoint on struct / class member
A symbolic watchpoint defined on a member variable for debugging is only valid as long as the expression is in scope. Once out of scope the watchpoint gets deleted.
When debugging some memory corruption we want to keep the watchpoint even the expression goes out of scope to find the location that overrides the variable and introduces the corruption.
(gdb) l
1 struct S { int v; };
2
3 void set(struct S* s, int v) {
4 s->v = v;
5 }
6
7 int main() {
8 struct S s;
9 set(&s, 1);
10 set(&s, 2);
11 set(&s, 3);
...
(gdb) s
set (s=0x7fffffffe594, v=1) at test.c:4
4 s->v = v;
# Define a new watchpoint on the member of the struct. The expression however
# is only valid in the current functions scope.
(gdb) watch s->v
Hardware watchpoint 2: s->v
(gdb) c
Hardware watchpoint 2: s->v
Old value = 0
New value = 1
set (s=0x7fffffffe594, v=1) at test.c:5
5 }
# The watchpoint gets deleted as soon as we leave the function scope.
(gdb) c
Watchpoint 2 deleted because the program has left the block in
which its expression is valid.
main () at test.c:10
10 set(&s, 2);
# Define the watchpoint on the location of the object to watch.
(gdb) watch -l s->v
# This is equivalent to the following.
(gdb) p &s->v
$1 = (int *) 0x7fffffffe594
# Define a watchpoint to the address of the member variable of the s instance.
# This of course only makes sense as long as the s instance is not moved in memory.
(gdb) watch *0x7fffffffe594
Hardware watchpoint 3: *0x7fffffffe594
(gdb) c
Hardware watchpoint 3: *0x7fffffffe594
Old value = 1
New value = 2
set (s=0x7fffffffe594, v=2) at test.c:5
5 }
(gdb) c
Hardware watchpoint 3: *0x7fffffffe594
Old value = 2
New value = 3
set (s=0x7fffffffe594, v=3) at test.c:5
5 }
Shell commands
# Run shell commands.
(gdb) shell zcat /proc/config.gz | grep CONFIG_KVM=
CONFIG_KVM=m
# Pipe gdb command to shell command.
(gdb) pipe info proc mapping | grep libc
0x7ffff7a1a000 0x7ffff7a42000 0x28000 0x0 r--p /usr/lib/libc.so.6
0x7ffff7a42000 0x7ffff7b9d000 0x15b000 0x28000 r-xp /usr/lib/libc.so.6
0x7ffff7b9d000 0x7ffff7bf2000 0x55000 0x183000 r--p /usr/lib/libc.so.6
0x7ffff7bf2000 0x7ffff7bf6000 0x4000 0x1d7000 r--p /usr/lib/libc.so.6
0x7ffff7bf6000 0x7ffff7bf8000 0x2000 0x1db000 rw-p /usr/lib/libc.so.6
Know Bugs
Workaround command + finish bug
When using finish inside a command block, commands after finish are not
executed. To workaround that bug one can create a wrapper function which calls
finish.
define handler
bt
finish
info reg rax
end
command
handler
end