Can realloc fail (return NULL) when trimming?

Question

If do the next:

int* array = malloc(10 * sizeof(int));

and them I use realloc:

array = realloc(array, 5 * sizeof(int));


        

Answer 1

The language (and library) specification makes no such guarantee, just like it does not guarantee that a "trimming" realloc will preserve the pointer value.

An implementation might decide to implement realloc in the most "primitive" way: by doing an unconditional malloc for a new memory block, copying the data and free-ing the old block. Obviously, such implementation can fail in low-memory situations.