Valgrind Tutorial - Part 2

By using valgrind we can also find out whether we are performing read/write operation on an array beyond its limit(size)

Take a look at this sample code

  int main(void)
{
      int i;
      int *a = malloc(sizeof(int) * 12);

     if (!a) return -1; /*malloc operation has failed*/

    for (i = 0; i < 15; i++){
        a[i] = i;
    }
   free(a);
   return 0;
}
  

You can see in the following code that iam allocating a integer array of size 12 by using malloc but i am trying to access beyond the size of array that is array[12],array[13] and array[14] which is wrong By using valgrind memcheck tool you can find out the error So first compile the code by passing -g to gcc to include debugging operation
 gcc -o app app.c -g

 Now run the valgrind tool using the following command
valgrind --tool=memcheck --leak-check=yes  ./app


==29768== Memcheck, a memory error detector
==29768== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==29768== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==29768== Command: ./app
==29768== 
==29768== Invalid write of size 4
==29768==    at 0x8048454: main (app.c:18)
==29768==  Address 0x41fa050 is 0 bytes after a block of size 40 alloc'd
==29768==    at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==29768==    by 0x8048428: main (app.c:13)
==29768== 
==29768== 
==29768== HEAP SUMMARY:
==29768==     in use at exit: 0 bytes in 0 blocks
==29768==   total heap usage: 1 allocs, 1 frees, 40 bytes allocated
==29768== 
==29768== All heap blocks were freed -- no leaks are possible
==29768== 
==29768== For counts of detected and suppressed errors, rerun with: -v

==29768== ERROR SUMMARY: 5 errors from 1 contexts (suppressed: 0 from 0)