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A summary of the official GDB documentation, made with love for https://secgroup.github.io/

GDB Cheat Sh*t

gdb [options] [PROGRAM [COREFILE or PID]] gdb [options] --args PROGRAM [INFARGS...] to pass any arguments after the executable file to the inferior

Options
  • --silent [or -q/--quiet] to start without printing the front material
  • --core COREFILE [or -c] to analyze a core dump
  • --pid PID [or -p] to debug a running process (as with the attach command)
  • --command EXECFILE [or -x] to execute commands from file (as with the source command)
  • --symbols SYMFILE [or -s] to read symbol table from file
Examples
  • gdb -q --args gcc -O2 -c foo.c
Examples of command-lists from a command file
b main
commands 1
  print argc
  continue
end

b *0xdeadbeef if x > 0
commands 2
  p i
  p b
  continue
end

run

Contents

2 Getting In and Out of gdb

2.2 Quitting gdb

  • quit [or q] to exit gdb. An interrupt (often Ctrl-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

2.3 Shell Commands

  • shell COMMAND [or !COMMAND] to invoke a standard shell to execute COMMAND

2.4 Logging Output

  • set logging on|off to enable/disable logging
  • set logging file FILEto change the name of the current logfile. The default logfile is gdb.txt

3 gdb Commands

3.3 Getting Help

  • help [or h] to display a short list of named classes of commands
  • help COMMAND to display a short paragraph on how to use that command
  • apropos ARGS to searche through all of the gdb commands and their documentation for the regular expression specified in ARGS
  • complete ARGS to list all the possible completions for the beginning of a command specified by ARGS
  • info [or i] to describe the state of your program. You can get a complete list of the info sub-commands with help info
  • info files [or info target] to display info on the debugged program (useful to find the entry point)
  • info functions [REGEXP] to list all defined functions or whose matching REGEXP
  • info address SYMBOL to find address of SYMBOL
  • info proc mappings to display the list of mapped memory regions
  • info registers [REGISTER] to display the contents of all the general-purpose processor registers or the content of register REGISTER
  • info sharedlibrary to display information about loaded libraries
  • info symbol ADDR to display the name of the symbol residing at a given address ADDR
  • info types [REGEXP] to display the list of types defined in the currently loaded modules or the list of types matching REGEXP
  • info variables [REGEXP] to display the list of global/static variables or whose matching REGEXP
Examples
  • help status
  • apropos alias
  • complete i
  • info addr system

4 Running Programs Under gdb

4.1 Compiling for Debugging

To request debugging information, specify the -g option when you run the compiler.

4.2 Starting your Program

  • run [or r] to start your program under gdb
  • start to set a temporary breakpoint at the beginning of the main procedure and then invoke the run command
  • set exec-wrapper WRAPPER to set the wrapper used to launch programs for debugging, with a shell command of the form exec WRAPPER program. You can use any program that eventually calls execve with its arguments as a wrapper
  • show exec-wrapper
  • set disable-randomization on|off to enable/disable address randomization
Examples
  • set exec-wrapper env 'LD_PRELOAD=custom_libc.so' (to pass an environment variable to the debugged program without setting the variable in your shell's environment)

4.3 Your Program's Arguments

  • set args to specify the arguments to be used the next time your program is run. If 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

4.4 Your Program's Environment

  • set environment VARNAME [VALUE] to set environment variable VARNAME to VALUE
  • show environment [VARNAME] to print the value of environment variable VARNAME. If VARNAME is not specified, print the names and values of all environment variables
  • unset environment [VARNAME] to remove variable VARNAME from the environment. If VARNAME is not specified, remove all environment variables
Examples
  • set environment LD_PRELOAD=./yourso.so

4.7 Debugging an Already-running Process

  • attach to attach to a running process started outside gdb

4.9 Debugging Multiple Inferiors and Programs

  • info inferiors to print a list of all inferiors currently being managed by gdb
  • inferior INFNO to make inferior number INFNO the current inferior
  • kill inferiors INFNO... to kill the inferior or inferiors identified by gdb inferior number(s)

4.10 Debugging Programs with Multiple Threads

  • thread THREADID to switch among threads
  • info threads to inquire about existing threads

4.11 Debugging Forks

  • set follow-fork-mode MODE to set the debugger response to a program call of fork or vfork. The MODE argument can be parent (the original process is debugged after a fork) or child (the new process is debugged after a fork)
  • show follow-fork-mode
  • set detach-on-fork MODE to detach one of the processes after a fork or retain debugger control over them both. The MODE argument can be on (the child process (or parent process, depending on the value of follow-fork-mode) will be detached and allowed to run independently) or off (both processes will be held under the control of gdb, one debugged and the other held suspended)
  • show detach-on-fork
  • set follow-exec-mode MODE to set debugger response to a program call of exec. The MODE argument can be new (gdb creates a new inferior and rebinds the process to this new inferior. The program the process was running before the exec call can be restarted afterwards by restarting the original inferior) or same (gdb keeps the process bound to the same inferior. The new executable image replaces the previous executable loaded in the inferior. Restarting the inferior after the exec call, with e.g., the run command, restarts the executable the process was running after the exec call)
  • show follow-exec-mode

4.12 Setting a Bookmark to Return to Later

  • checkpoint save a snapshot of the debugged program's current execution state
  • info checkpoints to list the checkpoints that have been saved in the current debugging session
  • restart CHKID to restore the program state that was saved as checkpoint number CHKID
  • delete checkpoint CHKID to delete the previously-saved checkpoint identified by CHKID

5 Stopping and Continuing

5.1 Breakpoints, Watchpoints, and Catchpoints

  • break [LOCATION] to a breakpoint at the given LOCATION. If LOCATION is not specified, set a breakpoint at the next instruction to be executed in the selected stack frame
  • break ... if COND to set a breakpoint with condition COND
  • tbreak ARGS to set a breakpoint enabled only for one stop (ARGS are the same as for the break command)
  • hbreak ARGS to set a hardware-assisted breakpoint (ARGS are the same as for the break command)
  • thbreak ARGS to set a hardware-assisted breakpoint enabled only for one stop (ARGS are the same as for the hbreak command)
  • rbreak REGEX to set breakpoints on all functions matching the regular expression REGEX
  • break ARGS thread THREADNO to set breakpoints on a particular thread
  • info breakpoints to print a table of all breakpoints, watchpoints, and catchpoints set and not deleted

Use a watchpoint to stop execution whenever the value of an expression changes.

  • watch EXPR to set a watchpoint that will break when the expression EXPR is written into by the program and its value changes
  • rwatch EXPR to set a watchpoint that will break when the value of EXPR is read by the program
  • awatch EXPR to set a watchpoint that will break when EXPR is either read from or written into by the program
  • info watchpoints to print a list of watchpoints

gdb sets a hardware watchpoint if possible. Hardware watchpoints execute very quickly, and the debugger reports a change in value at the exact instruction where the change occurs. If gdb cannot set a hardware watchpoint, it sets a software watchpoint, which executes more slowly and reports the change in value at the next statement, not the instruction, after the change occurs.

  • set can-use-hw-watchpoints 0|1 to set whether or not to use hardware watchpoints
  • show can-use-hw-watchpoints to show the current mode of using hardware watchpoints

In multi-threaded programs, watchpoints will detect changes to the watched expression from every thread.

  • delete [RANGE...] to delete the breakpoints, watchpoints, or catchpoints of the breakpoint ranges. If RANGE... is not specified, delete all breakpoints, watchpoints or catchpoints

  • disable [RANGE...] to disable the specified breakpoints. If RANGE... is not specified, disable all breakpoints

  • enable [RANGE...] to enable the specified breakpoints. If RANGE... is not specified, enable all breakpoints

  • enable once RANGE... to enable the specified breakpoints temporarily and then disable them after stopping your program

  • enable delete RANGE... to enable the specified breakpoints temporarily and then delete them after stopping your program

  • save breakpoints [FILE] to save breakpoint definitions to a file

Examples
  • watch x
  • watch *0x600850
  • watch *(int *)0x12345678 (to watch a 4-byte region at the specified address)
  • watch a*b + c/d
  • delete 1 2 3
  • delete 1-3 5-6
  • disable 1 2 3
  • enable delete 1 2

5.2 Continuing and Stepping

  • continue [or c] to resume program execution after a stop
  • finish to continue running until just after function in the selected stack frame returns
  • until [or u] to continue execution until the program counter is greater than the address of the jump (very useful to continue execution until loop exit)
  • advance LOCATION to continue running the program up to the given location
  • stepi [or si] to execute one machine instruction
  • nexti [or ni] to execute one machine instruction stepping over function calls

5.4 Signals

  • info signals [or info handle] to print a table of all the kinds of signals and how gdb has been told to handle each one
  • handle SIGNAL [KEYWORDS...] to change the way gdb handles signal SIGNAL. The keywords can be:
    • nostop to not stop your program when this signal happens
    • stop to stop your program when this signal happens. This implies the print keyword as well
    • print to print a message when this signal happens
    • noprint to not mention the occurrence of the signal at all. This implies the nostop keyword as well
    • pass [or noignore] to allow your program to see this signal
    • nopass [or ignore] to not allow your program to see this signal
Examples
  • handle SIGUSR1

6 Running Programs Backward

  • reverse-continue [or rc] to start executing in reverse beginning at the point where your program last stopped
  • reverse-stepi to reverse-execute one machine instruction
  • reverse-nexti to reverse-execute a single instruction in reverse (called functions are "un-executed" atomically)
  • reverse-finish to take you to the point where the current function was called

8 Examining the Stack

8.2 Backtraces

  • backtrace [N] [or bt] to print a backtrace of the entire stack
  • backtrace full [N] to print the values of the local variables also

8.3 Selecting a Frame

  • frame N [or f] to select frame number N (frame zero is the innermost (currently executing) frame)
  • frame STACKADDR to select the frame at address STACKADDR
  • up [N] to move N frames up the stack. N defaults to 1
  • down [N] to move N frames down the stack. N defaults to 1
  • select-frame [N] to silently select a stack frame

8.4 Information About a Frame

  • frame [or f] to print a brief description of the currently selected stack frame
  • info frame to print a verbose description of the selected stack frame
  • info args to print the arguments of the selected frame
  • info locals to print the local variables of the selected frame

10 Examining Data

  • print [/F] [EXPR] [or inspect] to evaluate and print the value of an expression of the language your program is written in. You can choose a different format by specifying /F, where F is a letter specifying the format. If you omit EXPR, gdb displays the last value again (useful to inspect the same value in an alternative format)
  • explore ARG to explore either an expression (in the source language), or a type visible in the current context of the program being debugged
Examples
  • p filename[0] = 'a'
  • p strlen(filename)
  • explore arr
  • explore struct ComplexStruct

10.3 Program Variables

Examples
  • p 'f2.c'::x (to refer to static variables)
  • p i@entry (to get value of variable i at the time the function got called)

10.4 Artificial Arrays

Examples
  • p *array@len
  • p/x (short[])0x12345678 (to create artificial arrays)

10.6 Examining Memory

  • x[/NFU] ADDR to examine memory. N, F, and U are all optional parameters that specify how much memory to display and how to format it
Examples
  • x/3uh 0x54320 (to display three halfwords (h) of memory, formatted as unsigned decimal integers (u), starting at address 0x54320)
  • x/4xw $sp (to print the four words (w) of memory above the stack pointer ($sp) in hexadecimal (x))
  • x/5i $pc-6
  • x/s *environ (to get the address of the first environment variable (or, alternatively, EBP of main + 16/32 bytes))

10.7 Automatic Display

  • display[/FMT] EXPR to add the expression EXPR to the list of expressions to display each time your program stops. FMT is used to specify a display format
Examples
  • display/i $pc

10.10 Value History

To refer to any previous value, use $ followed by the value's history number.

  • show values to print the last ten values in the value history
Examples
  • p *$

10.11 Convenience Variables

gdb provides convenience variables that you can use within gdb to hold on to a value and refer to it later. Convenience variables are prefixed with $.

Examples
  • set $foo = *object_ptr

10.12 Convenience Functions

Examples
  • print $_isvoid ($v)
  • p $_strlen($s)

10.13 Registers

  • info registers to print the names and values of all registers except floating-point and vector registers
  • info all-registers

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.

Examples
  • set $sp += 4

10.18 Copy Between Memory and a File

  • dump [FORMAT] memory FILE START_ADDR END_ADDR to dump the contents of memory from START_ADDR to END_ADDR, or the value of expr, to FILE in the given format
  • restore FILE [binary] BIAS START END to restore the contents of file FILE into memory

10.19 How to Produce a Core File from Your Program

  • generate-core-file [FILE] [or gcore] to produce a core dump of the inferior process

10.20 Character Sets

  • set charset CHARSET to set the current host and target character sets to CHARSET. If you type set charset <TAB><TAB>, gdb will list the names of the character sets that can be used for both host and target

10.22 Search Memory

  • find [/SN] START_ADDR, +LEN|END_ADDR, VAL1 [, VAL2, ...] to search memory for the sequence of bytes specified by VAL1, VAL2, etc. The search begins at address START_ADDR and continues for either LEN bytes or through to END_ADDR inclusive

17 Altering Execution

17.1 Assignment to Variables

set is really the same as print except that the expression's value is not printed and is not put in the value history.

Examples
  • print x=4
  • whatis width
  • set var width=47
  • set {int}0x83040 = 4

17.2 Continuing at a Different Address

  • jump LOCATION[or j] to resume execution at location. The jump command does not change the current stack frame, or the stack pointer, or the contents of any memory location or any register other than the program counter
Examples
  • jump *0x4028ba

17.3 Giving your Program a Signal

  • signal SIGNAL to resume execution where your program is stopped, but immediately give it the signal SIGNAL. The signal can be the name or the number of a signal
Examples
  • signal SIGINT
  • signal 2

17.4 Returning from a Function

  • return [EXPR] to discard the selected stack frame (and all frames within it). If you wish to specify a value to be returned, give that value as EXPR
Examples
  • return -1
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