Chapter 8. GNU Debugger (GDB)

Consider a source file named fibonacci.c that has the following contents:

#include <stdio.h>
#include <limits.h>

int main (int argc, char *argv[]) {
  unsigned long int a = 0;
  unsigned long int b = 1;
  unsigned long int sum;

  while (b < LONG_MAX) {
    printf("%ld ", b);
    sum = a + b;
    a = b;
    b = sum;
  }

  return 0;
}

Compile this program on the command line using GCC from Red Hat Developer Toolset with debugging information for the GNU Debugger:

$ scl enable devtoolset-11 'gcc -g -o fibonacci fibonacci.c'

This creates a new binary file called fibonacci in the current working directory.

Install debugging information for the glibc package:

# debuginfo-install glibc
Loaded plugins: product-id, refresh-packagekit, subscription-manager
--> Running transaction check
---> Package glibc-debuginfo.x86_64 0:2.17-105.el7 will be installed
...

This example assumes that you have successfully compiled the fibonacci binary file as shown in Example 8.1, “Compiling a C Program With Debugging Information”.

Start debugging fibonacci with gdb:

$ scl enable devtoolset-11 'gdb fibonacci'
GNU gdb (GDB) Red Hat Enterprise Linux 8.2-2.el7
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from fibonacci...done.
(gdb)

The fibonacci.c file listed in Example 8.1, “Compiling a C Program With Debugging Information” has exactly 17 lines. Assuming that you have compiled it with debugging information and you want the gdb utility to be capable of listing the entire source code, you can run the following command to change the number of listed lines to 20:

(gdb) set listsize 20

You can now display the entire source code of the file you are debugging by running the list command with no additional arguments:

(gdb) list
1       #include <stdio.h>
2       #include <limits.h>
3
4       int main (int argc, char *argv[]) {
5         unsigned long int a = 0;
6         unsigned long int b = 1;
7         unsigned long int sum;
8
9         while (b < LONG_MAX) {
10          printf("%ld ", b);
11          sum = a + b;
12          a = b;
13          b = sum;
14        }
15
16        return 0;
17      }

This example assumes that you have compiled the fibonacci.c file listed in Example 8.1, “Compiling a C Program With Debugging Information” with debugging information.

Set a new breakpoint at line 10:

(gdb) break 10
Breakpoint 1 at 0x4004e5: file fibonacci.c, line 10.

This example assumes that you have followed the instructions in Example 8.5, “Setting a New Breakpoint”.

Display the list of currently set breakpoints:

(gdb) info breakpoints
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x00000000004004e5 in main at fibonacci.c:10

This example assumes that you have compiled the fibonacci.c file listed in Example 8.1, “Compiling a C Program With Debugging Information” with debugging information.

Set a new breakpoint at line 7:

(gdb) break 7
Breakpoint 2 at 0x4004e3: file fibonacci.c, line 7.

Remove this breakpoint:

(gdb) clear 7
Deleted breakpoint 2

This example assumes that you have followed the instructions in Example 8.5, “Setting a New Breakpoint”.

Execute the fibonacci binary file:

(gdb) run
Starting program: /home/john/fibonacci

Breakpoint 1, main (argc=1, argv=0x7fffffffe4d8) at fibonacci.c:10
10          printf("%ld ", b);

This example assumes that you have followed the instructions in Example 8.8, “Executing the fibonacci Binary File” and the execution of the fibonacci binary stopped after reaching the breakpoint at line 10.

Display the current values of variables a and b:

(gdb) print a
$1 = 0
(gdb) print b
$2 = 1

This example assumes that you have followed the instructions in Example 8.8, “Executing the fibonacci Binary File”, and the execution of the fibonacci binary stopped after reaching the breakpoint at line 10.

Resume the execution:

(gdb) continue
Continuing.

Breakpoint 1, main (argc=1, argv=0x7fffffffe4d8) at fibonacci.c:10
10          printf("%ld ", b);

The execution stops the next time the breakpoint is reached.

Execute the next three lines of code:

(gdb) step 3
13          b = sum;

This allows you to verify the current value of the sum variable before it is assigned to b:

(gdb) print sum
$3 = 2