GNU Debugger

A set of notes and recipes on gdb, mainly related to C

Breakpoints

There are three ways to instruct GDB to pause execution of your program:

Configuration Files

GDB will always execute the commands in $HOME/.gdbinit before starting a debugging session. GDB will also look at $(pwd)/.gdbinit, which is useful for project specific configuration.

You can point GDB at a specific configuration file by running:

gdb -command=config-file <binary>

Commands

Information

Command Arguments Notes
print <variable> Print the value of a variable in the scope
print <array>@<size> Print a dynamically sized array by telling GDB the length in advance
print/x <variable> Same as print. Force hexadecimal output
print/c <variable> Same as print. Force character output
print/s <variable> Same as print. Force string output
print/f <variable> Same as print. Force float output
info locals Print all local variables
info threads Print information about all running threads
info breakpoints Print information about all breakpoints/watchpoints/catchpoint
info display Print information about all configured displays
backtrace - Print the backtrace up to the current function
show user List all the macros from the current user
help <topic> Get information about a certain topic or command
list <place> Show contextual code around a certain function, line number, etc
display <variable> Automatically display a variable at any breakpoint
undisplay - Undisplay all displayed variables
undisplay <id> Undisplay a displayed variable given a display ID
enable display <id> Enable a disabled display given its ID
disable display <id> Disable a display given its ID
frame <id> Print the value of a certain stack frame. 0 is the current one. Check backtrace. Printing a frame switches context to that frame
thread <id> Switch context to a certain thread
tty <path> Set the TTY GDB will use to print the debugged program’s output

The output of info breakpoints look like this:

Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x0000000100000f32 in main at main.c:9
        breakpoint already hit 1 time
2       hw watchpoint  keep y                      argc
3       breakpoint     keep y   0x0000000100000f3f in main at main.c:10
        stop only if argc > 2

Notice that this command also tells us how many times each breakpoint was hit.

Interaction

Command Arguments Notes
set variable <name> = <expression> Modify a local variable
set environment <name> = <expression> Set an environment variable
set $<name> = <expression> Set an convenience variable for use during the session

Flow

Command Arguments Notes
run - Run the program with the last passed arguments (defaults to none)
run <args...> Run the program with the given arguments
continue - Continue execution until a breakpoint is hit
continue <number> Continue execution ignoring the next N breakpoints
next - Execute the next line and pause
next <times> Execute next N number of times
step - Same as next, but enter the function given a line with a function call
finish - Resume execution until just after the current stack frame finishes. GDB will stop at any intermediary breakpoints
jump <place> Unsafely jump to a certain place in the code and continue execution from there

Breakpoints

Command Arguments Notes
delete - Delete all breakpoints
delete <id> Delete a breakpoint by its ID
disable - Disable all breakpoints
disable <id> Disable a breakpoint given its ID
enable <id> Enable a breakpoint given its ID
enable once <id> Enable a breakpoint given its ID once, and then disable it again
clear - Clear all breakpoints from the current line
clear <place> Clear all breakpoints from a certain line, function, etc
break <line> Add a breakpoint in a certain line of the current file
break <function> Add a breakpoint in a certain function of the current file
break <line>:<function> Add a breakpoint in a line of a certain file
break <file>:<function> Add a breakpoint in a function of a certain file
break +<offset> Add a breakpoint X lines after the current line
break -<offset> Add a breakpoint X lines before the current line
break <place> thread <thread> Add a breakpoint on a certain thread
break <place> if <expression> Create a conditional breakpoint. The expression may call local functions and GDB macros
break <place> thread <thread> if <expression> Create a conditional thread breakpoint
watch <variable> Stop when a certain variable changes. The watchpoint will be deleted if the variable goes out of scope
watch <expression> Stop when a certain expression is true, ie x < 5. The watchpoint will be deleted if the referenced variables go out of scope
tbreak <location> Set a temporary breakpoint that is deleted after hit once. Syntax is the same as break
condition <id> <expression> Turn a breakpoint, by its ID, into a conditional expression. Same as break <place> if <expression>
condition <id> Make a conditional breakpoint inconditional

Commands

Execute a set of commands everytime a certain breakpoint is hit. The syntax is:

command <breakpoint id>
<command 1>
<command 2>
...
<command N>
end

Shortcuts

Caveats

References