Debugging with GDB ( Stack and frames )

A debugger lets you pause a program, examine and change variables, and step through code. Spend a few hours to learn one so you can avoid dozens of hours of frustration in the future.

Getting Started: Starting and Stopping

• gcc -g myprogram.c
  • Compiles myprogram.c with the debugging option (-g). You still get an a.out, but it contains debugging information that lets you use variables and function names inside GDB, rather than raw memory locations (not fun).
• gdb a.out
  • Opens GDB with file a.out, but does not run the program. You’ll see a prompt (gdb) - all examples are from this prompt.
• r
• r arg1 arg2
• r <>
  • Three ways to run “a.out”, loaded previously. You can run it directly (r), pass arguments (r arg1 arg2), or feed in a file. You will usually set breakpoints before running.
• help
• h breakpoints
  • List help topics (help) or get help on a specific topic (h breakpoints). GDB is well-documented.
• q - Quit GDB

Stepping Through Code

Stepping lets you trace the path of your program, and zero in on the code that is crashing or returning invalid input.

• l
• l 50
• l myfunction
  • List 10 lines of source code for current line (l), a specific line (l 50), or for a function (l myfunction).
• next
  • Run program until next line, then pause. If the current line is a function, execute the entire function, then pause. Next is good for walking through your code quickly.
• step
  • Run the next instruction, not line. If the current instructions is setting a variable, it is the same as next. If it’s a function, it will jump into the function, execute the first statement, then pause. Step is good for diving into the details of your code.
• finish
  • Finish executing the current function, then pause (also called step out). Useful if you accidentally stepped into a function.

Breakpoints and Watchpoints

Breakpoints are one of the keys to debugging. They pause (break) a program when it reaches a certain location. You can examine and change variables, then resume execution. This is helpful when seeing why certain inputs fail, or testing inputs.

• break 45
• break myfunction
  • Set a breakpoint at line 45, or at myfunction. The program will pause when it reaches the breakpoint.
• watch x == 3
  • Set a watchpoint, which pauses the program when a condition changes (when x == 3 changes). Watchpoints are great for certain inputs (myPtr != NULL) without having to break on every function call.
• continue
  • Resume execution after being paused by a breakpoint/watchpoint. The program will continue until it hits the next breakpoint/watchpoint.
• delete N
  • Delete breakpoint N (breakpoints are numbered when created).

Setting Variables and Calling Functions

Viewing and changing variables at run-time is a huge part of debugging. Try giving functions invalid inputs or running other test cases to find the root of problems. Typically, you will view/set variables when the program is paused.

• print x
  • Print current value of variable x. Being able to use the original variable names is why the (-g) flag is needed; programs compiled regularly have this information removed.
• set x = 3
• set x = y
  • Set x to a set value (3) or to another variable (y)
• call myfunction()
• call myotherfunction(x)
• call strlen(mystring)
  • Call user-defined or system functions. This is extremely useful, but beware calling buggy functions.
• display x
• undisplay x
  • Constantly display value of variable x, which is shown after every step or pause. Useful if you are constantly checking for a certain value. Use undisplay to remove the constant display.

Backtrace and Changing Frames

The stack is a list of the current function calls - it shows you where you are in the program. A frame stores the details of a single function call, such as the arguments.

bt

Backtrace, aka print the current function stack to show where you are in the current program. If main calls function a(), which calls b(), which calls c(), the backtrace is

<= current location

b
a
main

• up
• down
  • Move to the next frame up or down in the function stack. If you are in c, you can move to b or a to examine local variables.
• return
  • Return from current function.

Crashes and Core Dumps

A “core dump” is a snapshot of memory at the instant the program crashed, typically saved in a file called “core”. GDB can read the core dump and give you the line number of the crash, the arguments that were passed, and more. This is very helpful, but remember to compile with (-g) or the core dump will be difficult to debug.

• gdb myprogram core
  • Debug myprogram with “core” as the core dump file.
• bt
  • Print the backtrace (function stack) at the point of the crash. Examine variables using the techniques above.

This is a quick guide, more information here:  

• Official Page - Documentation
• Sample session - Short Tutorial - Long Tutorial