gdb command

Debug programs interactively with the GNU Debugger.

Overview

GDB (GNU Debugger) is a powerful debugging tool that allows developers to monitor and control the execution of programs. It helps find and fix bugs by letting you pause execution, examine memory, set breakpoints, step through code, and inspect variables during runtime. GDB works with many programming languages including C, C++, Objective-C, and Fortran.

Options

-q, --quiet, --silent

Start GDB without printing the introductory and copyright messages

$ gdb -q ./program
(gdb)

-c FILE

Use FILE as a core dump to examine

$ gdb -c core ./program

-p PID

Attach to a running process with the specified process ID

$ gdb -p 1234

-x FILE

Execute GDB commands from FILE

$ gdb -x commands.gdb ./program

--args

Pass arguments after program name to the program being debugged

$ gdb --args ./program arg1 arg2

-d DIRECTORY

Add DIRECTORY to the path to search for source files

$ gdb -d /path/to/source ./program

Usage Examples

Basic debugging session

$ gdb ./program
(gdb) break main
Breakpoint 1 at 0x1149: file main.c, line 5.
(gdb) run
Starting program: /path/to/program 

Breakpoint 1, main () at main.c:5
5       int x = 10;
(gdb) next
6       printf("x = %d\n", x);
(gdb) print x
$1 = 10
(gdb) continue
Continuing.
x = 10
[Inferior 1 (process 12345) exited normally]
(gdb) quit

Debugging a program with arguments

$ gdb --args ./program input.txt output.txt
(gdb) run
Starting program: /path/to/program input.txt output.txt
[Program execution...]

Examining a core dump

$ gdb ./program core
(gdb) bt
#0  0x00007f8b4c5e32a3 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007f8b4c5e4921 in __GI_abort () at abort.c:79
#2  0x0000555555555160 in main () at crash.c:5

Tips:

Common GDB Commands

break (or b): Set a breakpoint at a line or function
run (or r): Start the program
continue (or c): Continue execution after stopping
next (or n): Execute next line without stepping into functions
step (or s): Execute next line, stepping into functions
print (or p): Print value of a variable or expression
backtrace (or bt): Show call stack
info breakpoints: List all breakpoints
watch: Set a watchpoint to stop when a variable changes

Using TUI Mode

Enable Text User Interface mode with Ctrl+X+A for a split view showing source code and GDB commands simultaneously.

Saving Breakpoints

Use save breakpoints file.txt to save your breakpoints to a file, and load them in future sessions with source file.txt.

Frequently Asked Questions

Q1. How do I start debugging a program?

A. Run gdb ./program, then use break main to set a breakpoint at the main function, and run to start execution.

Q2. How can I examine what caused a segmentation fault?

A. After the crash, use backtrace (or bt) to see the call stack and identify where the crash occurred. Then use frame N to select a specific frame and examine variables.

Q3. How do I debug a running process?

A. Use gdb -p PID to attach to a running process with the specified process ID.

Q4. How do I set conditional breakpoints?

A. Use break location if condition, for example: break main.c:25 if x > 10.

Q5. How do I print all elements of an array?

A. Use print *array@length where length is the number of elements to display.

References

https://sourceware.org/gdb/current/onlinedocs/gdb/

Revisions

2025/05/05 First revision