Is something wrong with getType(object)?

Question

For pset4, in this code(in main, to check collision of ball and anything else):

GObject object = detectCollision(window, ball);

        if (strcmp(getType(object), "GRect") == 0)
        {
            if (object == paddle)
            {
                velocityY *= -1;
            }
        }

On compiling returns, "Segmentation Fault, core dumped"

SO, I ran valgrind, and here's the result:

==4188== Memcheck, a memory error detector
==4188== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==4188== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==4188== Command: ./breakout
==4188== 
==4188== Invalid read of size 4
==4188==    at 0x804ECE3: getType (in /home/jharvard/Dropbox/pset4/breakout)
==4188==    by 0x8049786: main_ (breakout.c:94)
==4188==    by 0x804A0FD: main (in /home/jharvard/Dropbox/pset4/breakout)
==4188==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==4188== 
==4188== 
==4188== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==4188==  Access not within mapped region at address 0x0
==4188==    at 0x804ECE3: getType (in /home/jharvard/Dropbox/pset4/breakout)
==4188==    by 0x8049786: main_ (breakout.c:94)
==4188==    by 0x804A0FD: main (in /home/jharvard/Dropbox/pset4/breakout)
==4188==  If you believe this happened as a result of a stack
==4188==  overflow in your program's main thread (unlikely but
==4188==  possible), you can try to increase the size of the
==4188==  main thread stack using the --main-stacksize= flag.
==4188==  The main thread stack size used in this run was 8388608.
==4188== 
==4188== HEAP SUMMARY:
==4188==     in use at exit: 94,800 bytes in 549 blocks
==4188==   total heap usage: 612 allocs, 63 frees, 101,682 bytes allocated
==4188== 
==4188== LEAK SUMMARY:
==4188==    definitely lost: 8,717 bytes in 266 blocks
==4188==    indirectly lost: 78,182 bytes in 214 blocks
==4188==      possibly lost: 7,197 bytes in 67 blocks
==4188==    still reachable: 704 bytes in 2 blocks
==4188==         suppressed: 0 bytes in 0 blocks
==4188== Rerun with --leak-check=full to see details of leaked memory
==4188== 
==4188== For counts of detected and suppressed errors, rerun with: -v
==4188== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
Segmentation fault

I'm no expert at reading Valgrind, but as far as I understood, it means to tell me that something's wrong with getType through these lines:

==4188== Invalid read of size 4
==4188==    at 0x804ECE3: getType (in /home/jharvard/Dropbox/pset4/breakout)
==4188==    by 0x8049786: main_ (breakout.c:94)
==4188==    by 0x804A0FD: main (in /home/jharvard/Dropbox/pset4/breakout)
==4188==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

But I have checked that i'm not making any mistakes while using that, so what exactly is going on here ?

Thanks in advance

Answer 1

The function detectCollision() returns NULL if there's no collision happened. You have to check whether your object is equal to NULL before you pass it to getType().