GDB the debugger for C

13-03-2022

Why use a debugger

Debuggers are the best way for a programmer to find bugs. Print statements have been the first only way for most of us to debug our programs. However they are not as efficient as a debugger. With a debugger you can see the code executing live, you can inspect the value of variables, see which function called what, you can change values of variables on the fly and much more. Especially with C, where segmentation faults are not uncommon and where printf statements sometimes become useless, gdb is a must.

Basic commands

• b <line number/function name> new break point at line number or function. b test.c:100 breakpoint in file test.c line 100, b my_func breakpoint at my_func
• info b display breakpoints
• d <n> disable breakpoint n
• r <optional arguments> run program
• l list code
• p <var name><@n> to print var[with sequence of n elements from array].p *test print contents of pointer test, p arr@10 print 10 elements of array
• bt show backtrace (functions that called the current function and the one before etc)
• f <n> switch to frame n, used usually after bt to jump to previous function's context
• s step in (moves inside function under cursor or moves to next line)
• n continue to next line
• u step out (moves outside function or loop)
• ret <val> return from function with value val
• j <n> jump to line
• make run make
• dis disassemble
• c continue running code
• info local show values of variables in local context
• disp <var> display value of var after each line
• undisp stop displaying variable
• watch <var> break if var changes
• q quit
• info threads list threads
• thread <n> switch to thread n
• info args list agruments
• ni execute one machine instruction but don't step into functions
• si execute one machine instruction
• Pressing enter without typing anything executes previous command

Using gdb

First for gdb to be able to debug your c program you'll need to compile it with the -g or -ggdb flag which generates debugging symbols. Sometimes you might need to reduce the optimization from -O3 to -O0 so that code is less optimized but easier to debug otherwise some values might be optimized out. To run gdb run gdb ./my-program. You can also use the --tui flag which will also display your code at the same time.