Debugging with Emacs and GDB

Jun 24, 2015 · 3 minute read · Emacs

Introduction

Erroneous source code can be a nasty issue to tackle. Print statements that inform you about the states of variables are handy but sometimes they are unable to represent the intrinsic structure of the code. Debuggers allow you to run your code step by step, display and watch variables and see what is going on inside another program while it executes.

The GNU poject debugger

Here, we will use the GNU project debugger (GDB), because it is freely available, very actively developed and can be used on many operating systems. It can

start your program, specifying anything that might affect its behavior;
make your program stop on specified conditions;
examine what has happened, when your program has stopped;
change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.

GDB in Emacs

To debug our program, we will use Emacs and C source code, although gdb can be used from the command line and can debug many different languages.

A good overview about debugging in Emacs with GDB can be found in the Emacs manual.

A short step-by-step guide:

Open and save a new file hello.c with the following contents:

#include <stdio.h>

int main(int argc, char* argv[]) {
    int i;
    printf("hello world\n");
    for ( i=0;i<10;++i) {
        printf("%d \n",i);
    }
    return 0;
}

Compile it with M-x compile; enter the command gcc -Wall -g hello.c -o hello. Usually, it is very helpful to compile your source code with debugging flags (e.g., -g for gcc), so that nothing is optimized out by the compiler.
Activate the graphical debugging environment with M-x gdb-many-windows.
Start the debugger with M-x gdb; enter the command gdb -i=mi hello.

From withing the Grand Unified Debugger (GUD) buffer, you can run (r) the program to its end or to the first break point, start (start) and run in until the beginning of the main procedure. You can execute the program line-by-line (n) or step into functions with s.

Breakpoints can be set from withing the source buffer by clicking on the fringe or with C-x C-a C-b.

Customize GDB

To permanently set the nice GDB user interface layout, put

(setq gdb-many-windows t)

into your .emacs file. I have also written a function, that facilitates the starting of the debugger:

(defvar gdb-my-history nil "History list for dom-gdb-MYPROG.")
(defun dom-gdb-MYPROG ()
  "Debug MYPROG with `gdb'."
  (interactive)
  (let* ((wd "/path/to/working/directory")
         (pr "/path/to/executable")
         (dt "/path/to/datafile")
         (guess (concat "gdb -i=mi -cd=" wd " --args " pr " -s " dt))
         (arg (read-from-minibuffer "Run gdb (like this): "
                                    guess nil nil 'gdb-my-history)))
    (gdb arg)))

Window management

Upon debugging a program with many source files, GDB displays new source files in (random?) windows in your Emacs frame. This is especially tedious if you use gdb-many-windows. I have written a function dom-gdb-restore-windows, that resets the display and fixes the window layout: