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:

   run

   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:

   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:

   list

   The following output displays:

   12 int 13 main (void) 14 { 15 int b; 16 17 a = 3; 18 b = 4; 19 20 printf ("Hello, world!\n"); 21 22 foo (b); 23 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> 2 3 int a, c; 4 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:

   continue

   The following output displays:

   Continuing. 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:

    step

    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:

    backtrace

    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:

    quit