4.3.2. Running GDB

This section will describe a basic execution of GDB, using the following simple program:

#include <stdio.h>

char hello[] = { "Hello, World!" };

  fprintf (stdout, "%s\n", hello);
  return (0);

The following procedure illustrates the debugging process in its most basic form.

Procedure 4.1. Debugging a 'Hello World' Program

  1. Compile hello.c into an executable with the debug flag set, as in:
    gcc -g -o hello hello.c
    Ensure that the resulting binary hello is in the same directory as hello.c.
  2. Run gdb on the hello binary, that is, gdb hello.
  3. After several introductory comments, gdb will display the default GDB prompt:
  4. The variable hello is global, so it can be seen even before the main procedure starts:
    gdb) p hello
    $1 = "Hello, World!"
    (gdb) p hello[0]
    $2 = 72 'H'
    (gdb) p *hello
    $3 = 72 'H'
    Note that the print targets hello[0] and *hello require the evaluation of an expression, as does, for example, *(hello + 1):
    (gdb) p *(hello + 1)
    $4 = 101 'e'
  5. Next, list the source:
    (gdb) l
    1       #include <stdio.h>
    3       char hello[] = { "Hello, World!" };
    5       int
    6       main()
    7       {
    8         fprintf (stdout, "%s\n", hello);
    9         return (0);
    10      }
    The list reveals that the fprintf call is on line 8. Apply a breakpoint on that line and resume the code:
    (gdb) br 8
    Breakpoint 1 at 0x80483ed: file hello.c, line 8.
    (gdb) r
    Starting program: /home/moller/tinkering/gdb-manual/hello
    Breakpoint 1, main () at hello.c:8
    8         fprintf (stdout, "%s\n", hello);
  6. Finally, use the next command to step past the fprintf call, executing it:
    (gdb) n
    Hello, World!
    9         return (0);
The following sections describe more complex applications of GDB.