GDB Tutorial

I recommend using CGDB instead of GDB, as it is helpful to see what is visually taking place. CGDB uses GDB behind the scenes and shows you the code as it is being executed.

Launching

  1. To use GDB you must compile with the -g compiler flag. This is so the compiler and linker keep all the useful debugging information (symbol and code information).
  2. Launch GDB by telling it what program to debug: gdb myProgram or cgdb myProgram.

    NOTE: You could run your program at this point, but it won't be useful for debugging. You'd want to set a breakpoint first.

  3. If you want to run your program use the run command (shorthand: r).
  4. To quit GDB use the quit command (shorthand: q).

Breakpoints

Breakpoints can be set using the break command (shorthand: b).

There are multiple ways to set breakpoints, by:
Make things easier by setting a breakpoint before running your program.
  1. Set it on the first line using b 1
  2. Run your program using r.

By doing this you can step through your program one line at a time.

NOTE: A breakpoint stops execution before the line of code is executed.

Stepping

Step into the current line with the step command (shorthand: s) when execution is paused. For example, if execution is paused on the line x = foo();, the command s will enter foo() and break at the first line of foo().

Step over the current line with the next command (shorthand: n). It will execute all the code until it reaches the next line in the current scope. In the case of x = foo();, all of foo() will be executed and its returned value will be assigned to x. Execution will then pause on the line that comes after x = foo();.

Step out of a function with the finish command (shorthand: fin). It will finish the current function and pauses execution once the function is completed.

Complete execution of a loop from the last line of the loop using the until command and execution will resume on the line below it.

Resume normal execution with the continue command (shorthand: c); it'll stop at the next breakpoint.

Program State

View the value of a variable (when execution is paused) with the print foo command (shorthand: p foo). You can even view the result of an expression, such as p listContainsFoo().

Change the value of a variable with the set foo = 10 command.

Show the stack with the info stack command (shorthand: i s). View stack frame #3 with the frame 3 command.

Advanced Breakpoints

Conditional breakpoints are used to pause execution only if some condition is met.

You can create them with expressions. For example, b 6 if x == 0 or b 6 if listContainsFoo().

See the list of current breakpoints with the info breakpoints command (shorthand: i b). NOTE: Each breakpoint is given a number.

Delete a breakpoint in two ways:

Temporarily disable a breakpoint with the disable 1 command (shorthand: dis 1).

Re-enable breakpoints with the enable 1 command (shorthand: en 1).

CAUTION: The clear command shorthand is cl. The continue command shorthand is c. You do not want to mix up these two.

CGDB

If you are using CGDB know these shorthand commands:

Escape key: move cursor to source code view
↑ and ↓ keys: scroll through the code

i key: move the cursor to the GDB view
↑ and ↓ keys: scroll through GDB command history

Credits

Written by Chris Blume

Edited by Contented Design

Designed by San Chung