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GDB step-by-step debugging

14 January 2024 10 min read C / C++ Open source

GDB (the GNU Debugger) is the standard debugger for C and C++ programs on Linux. This tutorial walks through the most useful commands with a concrete example.

All code for this tutorial is on GitHub: github.com/achint/gdb-tutorial

Compiling for debugging

You must compile with the -g flag to include debug symbols. Without it, GDB can't map machine instructions back to your source lines.

gcc -g -o myprogram myprogram.c

For a release build without debug info:

gcc -O2 -o myprogram myprogram.c

Starting GDB

gdb ./myprogram

You're now in the GDB shell. Type run (or r) to start the program.

Setting breakpoints

(gdb) break main          # break at the start of main()
(gdb) break myfile.c:42   # break at line 42 of myfile.c
(gdb) break my_function   # break at the start of a function

Stepping through code

Command	Short	What it does
`next`	`n`	Execute the next line (steps over function calls)
`step`	`s`	Step into function calls
`continue`	`c`	Run until the next breakpoint
`finish`	`fin`	Run until the current function returns

Inspecting variables

(gdb) print my_variable     # print the value of a variable
(gdb) print *my_pointer     # dereference and print a pointer
(gdb) info locals           # print all local variables
(gdb) info args             # print all function arguments

Reading the stack

(gdb) backtrace             # show the call stack
(gdb) frame 2               # switch to frame 2
(gdb) info frame            # details about the current frame

Watchpoints

Watchpoints pause execution whenever a variable's value changes — useful for tracking down unexpected mutations.

(gdb) watch my_variable     # pause when my_variable changes
(gdb) rwatch my_variable    # pause when my_variable is read