I'm kind of at a loss about why it can return either of those two things, somebody want to take a shot at explaining it?
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> I'm kind of at a loss about why it can return either of those two things, somebody want to take a shot at explaining it?
Any return from malloc, whether it succeeds or not, is a valid argument to `free()`. Hence, it can return NULL because `free(NULL)` is legal, and anything other than NULL has to be a unique pointer, because if it returns a duplicate then calling `free()` no the duplicate will crash.
Answers to this question taught me about the candidates taste and understanding of good API design :-).
Both NULL and "unique pointer" are correct answers, but all modern implementations only chose to return one of these. My follow-up question is "why ?" :-).
However, IIRC the only malloc error is ENOMEM. It's unclear why malloc(0) would run into that. (If malloc(0) did run into an ENOMEM, then NULL would be required, but the result of malloc(0) need not tell you anything about subsequent calls to *alloc functions. However, there is a possible malloc guarantee to consider.)
There's some interaction with realloc() which may favor NULL or a non-null pointer to 0 bytes, but that's too much work to figure out now.
Suppose that malloc(0) returns a not-NULL value. Is malloc(0) == malloc(0) guaranteed to be false? (I think that it is, which is how ENOMEM can happen.)
So, the "right" answer is probably malloc(0) returns a unique pointer because then the error check is simpler - a NULL return value is always a true error, you don't have to look at size.
malloc() is a very old API. A more modern version would probably looks like:
err_code malloc(size_t size, void **returned_ptr);
The current malloc() overloads the return to say NULL == no memory available/internal malloc fail for some reason and as far as the standard goes, allowing NULL return if size==0.
So if you get NULL back from malloc, did it really mean no memory/malloc fail, or zero size passed in ?
glibc and all implementations distinguish the two by allocating a internal malloc heap header, but with internal size bookkeeping size of zero, returning a pointer to the byte after the internal malloc heap header.
The only valid things you can do with the returned pointer is test it for != NULL, or pass it to realloc() or free(). You can never dereference it.
Returning a valid pointer to NO DATA is what all modern implementations do when a size==0 is requested.
In an interview situation, discussions around all these points are very productive, telling me how the candidate thinks about API design and how to fix a bad old one, whether they know anything about malloc internals (which is essential as overwriting internal malloc header info is a common security attack), and how they deal with errors returned from code.
Remember, it was only my warmup question :-). Things get really interesting after that :-) :-).
