GDB Getting started with GDB Working with a Core File

Example

Created this really bad program

 #include <stdio.h>
 #include <ctype.h>

// forward declarations


void bad_function()
{

   int *test = 5;

   free(test);

}


int main(int argc, char *argv[])
{
   bad_function();
   return 0;
}


gcc -g ex1.c

./a.out   //or whatever gcc creates
Segmentation fault (core dumped)

gdb -c core a.out

Core was generated by `./a.out'.

Program terminated with signal SIGSEGV, Segmentation fault. #0 __GI___libc_free (mem=0x5) at malloc.c:2929 2929 malloc.c: No such file or directory.

(gdb) where

#0 __GI___libc_free (mem=0x5) at malloc.c:2929 #1 0x0000000000400549 in bad_function () at ex1.c:12 #2 0x0000000000400564 in main (argc=1, argv=0x7fffb825bd68) at ex1.c:19

Since I compiled with -g you can see that calling where tells me that it didn't like the code on line 12 of bad_function()

Then I can examine the test variable that I tried to free

(gdb) up

#1 0x0000000000400549 in bad_function () at ex1.c:12 12 free(test);

(gdb) print test

$1 = (int *) 0x5

(gdb) print *test

Cannot access memory at address 0x5

In this case the bug is pretty obvious I tried to free a pointer that was just assigned the address 5 which wasn't created by malloc so free has no idea what to do with it.