GDB

GDB (GNU Project Debugger): Lets you look inside programs and debug them.

Three Types of Errors:

1. Syntax Errors: Occur during compilation.
2. Runtime Errors: Occur while the program is executing.

• Typically prevents program from continuing.

3. Logical Errors: Occur while the program is executing.

• Typically allows program to keep going.

Debugger Terms:

• Step into goes into the method and executes each piece step-by-step.
• Step over executes the method as one step.
• Step return runs the remainder of the current method as one step.
• Resume leaves the debugger.
• Line breakpoint temporarily pauses execution at this line.

Common GDB Commands

Basic Execution:

• break: Place a breakpoints.
• continue: Continue executing until the next break point.
• next: Execute the next line as a single instruction.
  • Any functions in the next line will be executed in a single step.
• step: Execute the next line, stepping into any functions.
• finish: Execute until current function returns (stepping out).
• print: Print the value of a variable.

Manipulation:

• set var <variable_name>=value: Set the value of a variable.
• return <expression>: Force the current function to return immediately with the given value.

Help:

• help: e.g., help all

Demo: Debugging a Program with GDB

Here is a program with a logical error:

#include <stdio.h>
int main()
{
    int i, num, j;
    printf("Enter a number: ");
    scanf("%d", &num);
    for (i=1;i<num;i++)
        j=j*i;
    printf("Factorial of %d is %d\n", num, j);
}

Here is an example interaction showing the error:

$ ./factorial
Enter a number: 6
Factorial of 6 is 3931680

To debug our program we need to compile our program with the -g flag before running it with gdb.

$ $ gcc -o factorial -std=c99 -g factorial.c
$ gdb --quiet factorial
Reading symbols from factorial...
(gdb) break 7
Breakpoint 1 at 0x119b: file factorial.c, line 7.
(gdb) run
Starting program: /tmp/temporary/factorial 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Enter a number: 7

Breakpoint 1, main () at factorial.c:7
7               for (i=1;i<num;i++)
(gdb) print j
$1 = 32767

• Here we set a breakpoint on line 7, run the program up until that breakpoint, and print the value of j (the variable supposed to store the result of the factorial calculation).
  • We can see that j has a garbage value in it (32767). To fix the program we just need to initialize j with a value of 1
    • We could also manipulate the value of j inside gdb (e.g., set var j=1)

Note: If compiled with -Wall, the following warning is given, warning us about j being uninitialized.

hello.c:6:9: warning: ignoring return value of ‘scanf’ declared with attribute ‘warn_unused_result’ [-Wunused-re
sult]
    6 |         scanf("%d", &num);
      |         ^~~~~~~~~~~~~~~~~
In file included from /usr/include/stdio.h:906,
                 from hello.c:1:
In function ‘printf’,
    inlined from ‘main’ at hello.c:9:2:
/usr/include/bits/stdio2.h:86:10: warning: ‘j’ may be used uninitialized [-Wmaybe-uninitialized]
   86 |   return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
hello.c: In function ‘main’:
hello.c:4:21: note: ‘j’ was declared here
    4 |         int i, num, j;
      |                     ^

Example: Playing with gdb

$ gdb --quiet factorial
Reading symbols from factorial...
(gdb) break 6
Breakpoint 1 at 0x1180: file factorial.c, line 6.
(gdb) run
Starting program: /tmp/temporary/factorial 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Breakpoint 1, main () at factorial.c:6
6               scanf("%d", &num);
(gdb) print num
$1 = 32767
(gdb) next
Enter a number: 42
7               for (i=1;i<num;i++)
(gdb) print num
$2 = 42

• (gdb) break 6: Set a breakpoint on the line containing scanf
• (gdb) run: Run the program until the next breakpoint.
• (gdb) print num: Print the value of num. We can see that it returns a garbage value (32767).
• we enter the number 42
• (gdb) print num: Print the value of num. We can see that it returns the value of 42.