Debugging

[cecil_rabmere]

What flags do I compile my program with so I can debug it? And how do I run the debugger from a unix terminal? Can anyone direct me to a site with the basic commands once running the debugger? Need to trace a segmentation fault, thanks!

[dwks]

What flags do I compile my program with so I can debug it?

With gcc, add -g:

gcc -g program.c -o program

You can also get useful warnings like so:

gcc -W -Wall -ansi -pedantic -g program.c -o program

And how do I run the debugger from a unix terminal?

You can run gdb like this.

gdb program

After compiling "program", of course.

Can anyone direct me to a site with the basic commands once running the debugger?

http://www.bgsu.edu/departments/compsci/docs/gdb.html
Just search for "gdb commands" or some such.

Need to trace a segmentation fault, thanks!

Good luck . . . you can post the code, if you want to.

[dwks]

A quick example:

$ cat seg.c
#include <stdio.h>

int get_num(int array[], int x) {
    return 2 * array[x];
}

int main() {
    int array[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    int x, sum = 0;

    for(x = 0; x < (int)(sizeof array); x --) {  /* oops */
        sum += get_num(array, x);
    }

    return 0;
}
$ gcc -g seg.c -o seg
$ ./seg
Segmentation fault
$ gdb ./seg
GNU gdb 6.7.1-debian
Copyright (C) 2007 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-linux-gnu"...
Using host libthread_db library "/lib/libthread_db.so.1".
(gdb) run
Starting program: seg 

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400460 in get_num (array=0x7fffb25329a0, x=-19049) at seg.c:4
4	    return 2 * array[x];
(gdb) backtrace
#0  0x0000000000400460 in get_num (array=0x7fffb25329a0, x=-19049) at seg.c:4
#1  0x00000000004004d0 in main () at seg.c:12
(gdb) list
1	#include <stdio.h>
2	
3	int get_num(int array[], int x) {
4	    return 2 * array[x];
5	}
6	
7	int main() {
8	    int array[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
9	    int x, sum = 0;
10	
(gdb) frame 1
#1  0x00000000004004d0 in main () at seg.c:12
12	        sum += get_num(array, x);
(gdb) list
7	int main() {
8	    int array[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
9	    int x, sum = 0;
10	
11	    for(x = 0; x < (int)(sizeof array); x --) {  /* oops */
12	        sum += get_num(array, x);
13	    }
14	
15	    return 0;
16	}
(gdb) p array
$1 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
(gdb) p x
$2 = -19049
(gdb) quit
The program is running.  Exit anyway? (y or n) y
$