CODEWITHSUNDEEP
cwindowsmemory-management
Anish Ramaswamy
Anish Ramaswamy

Reputation: 2351

c - realloc() succeeds in Windows 7 but fails in Windows XP, why?

So this code of mine had the following realloc block:

char **ptr = NULL;

void realloc_ptr(unsigned int new_size)
{    
    void *temp = NULL;

    temp = realloc(ptr, new_size * sizeof(*ptr));

    if(temp != NULL) {
        ptr = temp;
    }
    else {
        exit(EXIT_FAILURE);
    }
}

new_size is incremented right before this function is called. This array is always only expanded. Also, new_size never exceeds 3 in my code (for now).

Now the above realloc call worked fine during my testing in Windows 7. When I tested this code in XP, the above code would succeed 3 times and then it would throw an exception on the 4th time. I'll post the exact exception when I get back to the code (this computer doesn't have the code).

I'm guessing that my memory is too fragmented and the system could not allocate a contiguous block of memory. I have tested my code for memory leaks and fixed all of them (I hope). Any ideas why this is happening?

EDIT:

This above problem went away when I used Doug Lea's malloc.c. But I still want to know why this happened.

Thanks!

Upvotes: 2

Views: 333

Answers (2)

Alexey Frunze
Alexey Frunze

Reputation: 62106

When I tested this code in XP, the above code would succeed 3 times and then it would throw an exception on the 4th time. I'll post the exact exception when I get back to the code (this computer doesn't have the code).

That (the exception) means you either have invalid pointers (or indices) somewhere (perhaps, uninitialized) and/or a memory corruption. realloc() must fail silently and return NULL if the request cannot be satisfied.

And it's perfectly normal to have different behavior on different OSes.

Upvotes: 2

Bernd Elkemann
Bernd Elkemann

Reputation: 23560

My best guess is that new_size is just too large to be available in contiguous virtual memory. You can never ever be sure that a large amount of contiguous virtual memory is available on 32-bit systems.

You generally should try to use multiple smaller blocks and if possible not even alloc them all at the same time but alloc'ing the next after the previous was free'ed.

Upvotes: 2

Related Questions