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GDB: The Gnu Debugger

My video on GDB basics

The Gnu Debugger, or GDB, is a debugger for a ton of different languages, but what we care about is that it can debug C. gdb is the text-based debugger, but numerous implementations exist that add a graphical interface. In this tutorial we'll use the text-based debugger because

  • it is my preference since it involves less clicking and moving the mouse around (yes, I'm that lazy), and
  • if you can use the command line tool you will understand how to use it in your IDE if you figure out where all the buttons are, but I couldn't possibly demo every single GUI out there in one tutorial!

Read all about it on their website!

To use GDB

From the command line, after compiling with debugging symbols (-g flag in GCC), use either of the following. The latter will allow you to pass arguments to your program:

gdb <executable>
gdb --args <executable> <args...>

Overview of useful commands

Many of these have shortened versions! You'll probably recognize their functionality from other debuggers you've used.

Command Abbrev. Action
help <c> h Help info about command c
file <b> Reload the symbol table after recompiling the binary b to debug a modified program without quitting GDB
directory Update the list command to match recompiled binary; run after file <b>
break <n> b Insert breakpoint at line n/function named "n"
break <f>::<n> b Insert breakpoint at line n/function named "n" of file "f" (no file extension)
disable <#> dis Temporarily disable breakpoint #
enable <#> en Re-enable breakpoint #
delete <#...> d Delete breakpoint(s) #
delete d Delete all breakpoints
condition <#> <e> cond Stop at breakpoint # if expression e is true
run r (Re)start the program until next breakpoint, crash, or end
continue c Continue until next breakpoint, crash, or end
next n Go to next instruction
step s Step into a function
up Go up (to caller)
down do Go back down (call after up)
list l Show surrounding code around current line
print <e> p Print expression (identifier, or evaluate e)
record / record btrace Enable rewinding from this point on in an already running program (not available on all environments; requisite for following commands)
reverse-continue rc Go back to point at which program last stopped
reverse-step rs Step backwards into the last executed line, if it was a function
reverse-next rn Go back one line
quit q Quit GDB