Using realloc to shrink memory allocation

Question

I want to use realloc to free memory from the end of a chunk of memory. I understand that the standard does not require that realloc succeed, even if the memory requested is lower than the original malloc / calloc call. Can I just realloc , and then if it fails returns the original?

// Create and fill thing1
custom_type *thing1 = calloc(big_number, sizeof(custom_type));
// ...

// Now only the beginning of thing1 is needed
assert(big_number > small_number);
custom_type *thing2 = realloc(thing1, sizeof(custom_type)*small_number);

// If all is right and just in the world, thing1 was resized in-place
// If not, but it could be copied elsewhere, this still works
if (thing2) return thing2;

// If thing2 could not be resized in-place and also we're out of memory,
// return the original object with extra garbage at the end.
return thing1;

This is not a minor optimization; the part I want to save might be as little as 5% of the length of the original, which could be several gigabytes.

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Note: Using realloc to shrink the allocated memory and Should I enforce realloc check if the new block size is smaller than the initial? are similar but do not address my particular question.

Answer 1

Yes, you can. If realloc() is not successful, the original memory region is left untouched. I usually use code like this:

/* shrink buf to size if possible */
void *newbuf = realloc(buf, size);
if (newbuf != NULL)
    buf = newbuf;

Make sure that size is not zero. The behaviour of realloc() with a zero-length array depends on the implementation and can be a source of trouble. See this question for details.