Is it safe to "half free" a chunk of memory to shrink it?

Question

I have been told that even if it were safe it's definitely bad practice. My question is, is it actually safe to do this, or is it down-right undefined behavior? Suppose I have allocated a 100 byte chunk of memory, but I want to shrink it to 50. Would it be safe to add 50 to the pointer and free it? Here is an example in code:

char *ptr = malloc(100);
//...
free(ptr+50);
//Presumably, since I freed the second half only, would that be equivalent to ptr = realloc(ptr, 50);?

Is this cross-platform/undefined behavior? Is it safe to use this method of shrinking memory?

Answer 1

You can only pass to free a pointer that was returned from malloc, realloc, calloc. Passing anything else is undefined behavior.

Section 7.22.3.3p2 of the C standard regarding the free function states:

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by a memory management function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

The proper thing to do here would be to allocate a fresh block of memory, use memcpy to copy the bytes over, and free the old block.