Why do I get a C malloc assertion failure?

Question

I am implementing a divide and conquer polynomial algorithm so I can benchmark it against an OpenCL implementation, but I can't get malloc to work. When I run the program, it allocates a bunch of stuff, checks some things, then sends the size/2 to the algorithm. Then when I hit the malloc line again it spits out this:

malloc.c:3096: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed.
Aborted

The line in question is:

int *mult(int size, int *a, int *b) {
    int *out,i, j, *tmp1, *tmp2, *tmp3, *tmpa1, *tmpa2, *tmpb1, *tmpb2,d, *res1, *res2;
    fprintf(stdout, "size: %d\n", size);

    out = (int *)malloc(sizeof(int) * size * 2);
}

I checked size with a fprintf, and it is a positive integer (usually 50 at that point). I tried calling malloc with a plain number as well and I still get the error. I'm just stumped at what's going on, and nothing from Google I have found so far is helpful.

Any ideas what's going on? I'm trying to figure out how to compile a newer GCC in case it's a compiler error, but I really doubt it.

Answer 1

99.9% likely that you have corrupted memory (over- or under-flowed a buffer, wrote to a pointer after it was freed, called free twice on the same pointer, etc.)

Run your code under Valgrind to see where your program did something incorrect.