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debug-tools

Getting aquainted with gdb

Main gdb commands

• r : run or relaunch execution.

Navigation

• n : step over the next line
• s : step into the next line, dive into a function call.
• c : continue execution until next breakpoint or regular interruption.

Breakpoints

• b : set a breakpoint to the line you are on.
• b [func_name] : set a breakpoint to that function first line.
• b [source_file_name:line_number] : set a breakpoint to that source file line.

Visualisation

• p [variable] : prints a variable once. The C syntax fully applies so you can cast and dereference those, as well as try some dereferencing syntax. Great to fix, understand and test your pointers on pointers !
• disp [variable] : keeps printing a variable throughout a function exec, even when you come back to this function.
• bt : Backtrace, shows and numbers the stack frames. Allowing you to navigate in or out function calls / down or up the stack. Makes fixing segfaults a breeze !
• f [stack number] : allows you to dive into any stack frame / function call.

How to use gdb to debug child process forks

• Launch gdb with your program : gdb ./minishell
• set follow-fork-mode child
• Set a breakpoint in a function inside the child : b child_proc
• Open the text user interface : tui e
  • Tui display may get scrambled when your program outputs on stdout, don't panic and just refresh your display by scrolling inside the code preview window a bit.
• Run r
• When gdb meets a fork its sends a SIGTRAP signal, continue to resume execution : c