GDB Tutorial

gdb Cheatsheet#

Launching GDB#

Using GDB#

After you have launched GDB successfully, you can run the following commands to start, control, inspect and stop the program you are debugging.

• Stack & Frame Commands

  Command	Syntax	Behaviour
  Frame Selection	frame n	Select a stack frame, where Frame 0 is the innermost (currently executing) Frame, Frame 1 is the frame that called Frame 0, Frame 2 is the frame that called Frame 1 and so on. The highest numbered Frame in the stack trace is the main function.
  	f n
  	up {n}	Move up n frames. If n is omitted, n defaults to 1. Positive values of n will move to the outermost frames.
  	down {n}	Move down n frames. If n is omitted, n defaults to 1. Positive values of n will move to the innermost frames.
  Viewing Backtraces	backtrace	Print a backtrace of the entire stack, one line per frame for all frames in the stack.
  	backtrace n	Print a backtrace of the innermost n frames
  	backtrace -n	Print a backtrace of the outermost n frames
  Frame Information	info frame	Display information about the currently selected stack frame.
  	info args	Display arguments of the currently selected frame
  	info locals	Display variables local to the currently selected frame

• Running Programs

  Command	Syntax	Behaviour
  Starting & Stopping Programs	run	Run your program under GDB
  	kill	Kill the process running under GDB
  	Ctrl + C	...
  Managing Program Arguments	set args strings	Set the arguments to be used when your program is next ran (using the run command). strings is a list of string arguments.
  	show args strings	Show the arguments passed to the program when it is run.

• Stepping

  Command	Syntax	Behaviour
  Resuming Execution	continue	Continue program execution until the next breakpoint is hit.
  	c
  Stepping (Source Code)	step {count}	Continue running your program until control reaches a different source line, then stop it and return control to GDB. If count is included, GDB will step count times before stopping. If count is omitted, defaults to 1.
  	s {count}
  	next {count}	Continue running your program until control reaches a different source line in the innermost stack frame. If the current frame contains a call to another function, it will execute the entire function before stopping. If count is included, GDB will step count times before stopping. If count is omitted, defaults to 1.
  	n {count}
  	finish	Continue running until just after the function in the currently selected stack frame returns.
  	fin
  Stepping (Machine Instructions)	stepi {count}	Execute one machine instruction, then stop and return to the debugger. Optional argument count behaves identically as described in source code stepping.
  	si {count}
  	nexti {count}	Execute one machine instruction, skipping over function calls. This is similar to how next operates. Optional argument count behaves identically as described in source code stepping.
  	ni {count}

• Examining Memory

  Command	Syntax	Behaviour
  Examing Memory	x addr	Print memory starting at address addr
  	x{/nfu} addr	Print n counts of u unit size starting at address addr with the display format specified by f. The possible values for f and u are explained below.

• Examining Variables & Registers

  Command	Syntax	Behaviour
  Inspecting Variables	info locals	Show all local variables of the current stack frame.
  	print {/f} expr	Computes the value of the expression expr and prints the result. expr can be any valid C expression, such as the name of variable in your program. The optional parameter /f is one of the format specifiers described in Format Specifiers.
  	p {/f} expr
  Inspecting Registers	info registers	Display a list of integer registers and their values in the current stack frame.
  	info r
  	info register reg	Display the value of register reg in the current stack frame.
  	info r reg

Format Specifiers#

Unit Sizes#

Other Resources#

This is not an exhaustive cheatsheet. GDB is a very complex debugger with a lot of additional functionality we haven’t explained here. The commands listed above will be those you need to use most frequently.

If you would like to learn more about GDB, you can check out the GDB Documentation. This is a very detailed guide to GDB.

lldb#

If you are using macOS, you will have to use a different debugger - lldb. Both debuggers offer similar functionality, but you will have to learn an alternate command syntax. Fortunately, the lldb documentation has a page dedicated to listing the lldb equivalent of many of the above gdb commands available here.