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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

3.4. Debug the Executable

To start GDB, use the following commands:

gdb hello

After the initial copyright and configuration information, GDB returns its own prompt, (gdb). The following is a sample debugging session:

  1. To set a breakpoint, type:

    break main

    The following output displays:

    Breakpoint 1 at 0x132: file hello.c, line 15.

    Note the exact address and line number may vary, depending upon the target architecture being debugged and the exact layout of the C code in the hello.c file.

  2. To run the program, type:


    The following output displays (when the program stops at a breakpoint):

        Starting program: hello
        Breakpoint 1, main () at hello.c:15
        15 a = 3;

    Again note that the exact line number and instruction displayed is dependent upon the source code layout.

  3. To print the value of variable, a, type:

    print a

    The following output displays:

    $1 = 0
  4. To execute the next command, type next:


    The following output displays:

        16 b = 4;
  5. To display the value of a again, type:

    print a

    The following output displays:

        $2 = 3
  6. To display the program being debugged, type:


    The following output displays:

    12         int
    13         main (void)
    14         {
    15           int b;
    17           a = 3;
    18           b = 4;
    20           printf ("Hello, world!\n");
    22           foo (b);
    24           return 0;
    25         }
  7. To list a specific function code, use the list command with the name of the function to be display. For example, type:

    list foo

    The following output displays:

     1          #include <stdio.h>
     3          int a, c;
     5          static void
     6          foo (int b)
     7          {
     8            c = a + b;
     9            printf ("%d + %d = %d\n", a, b, c);
    10          }
  8. To set a breakpoint at line seven, enter the following input (set a breakpoint at any line by entering break linenumber, where linenumber is the specific line number to break):

    break 8

    The following output displays:

    Breakpoint 2 at 0xf4: file hello.c, line 8.
  9. To resume normal execution of the program until the next breakpoint, type:


    The following output displays:

    Hello, world!
    Breakpoint 2, foo (b=4) at hello.c:8
    8 c = a + b;
  10. To step to the next instruction and execute it, type:


    The following output displays:

    9 printf ("%d + %d = %d\n", a, b, c);
  11. To display the value of c, type:

    print c

    The following output displays:

    $3 = 7
  12. To see how you got to where you are, type:


    The following output displays:

    #0 foo (b=4) at hello.c:9
    #1 0x15c in main () at hello.c:18
  13. To exit the program and quit the debugger, type:


  Published under the terms of the GNU General Public License Design by Interspire