GDB
GDB (the GNU Debugger) lets you inspect and control a running program: pause execution at any line, examine variables, walk the call stack, and step through code. It works on C, C++, Fortran, and other compiled languages by reading debug symbols embedded during compilation.
Instead of scattering printf statements and recompiling after every guess, GDB lets you attach to a running process or post-mortem core dump and ask questions directly: what's in this variable, how did we get here, what did the caller pass in?
GDB requires debug information to be useful. Compile with -O0 -g for reliable debugging (no optimization, keep debug symbols). Binaries compiled with -O3 and no -g are nearly impossible to debug—the optimizer eliminates the structure GDB needs.
$ gcc -O0 -g -o app app.c # compile with debug symbols $ gdb ./app # start debugger (gdb) break main # set breakpoint (gdb) run # run until breakpoint (gdb) next # step over next line (gdb) print variable # print variable value (gdb) quit # exit GDB
GDB is command-line based. Commands can be abbreviated (e.g., b for break, c for continue, p for print). Press Enter to repeat the last command.