malloc(0) actually works? [duplicate]

Answer 1

It is undefined behavior to dereference the pointer returned by malloc(0).

From the C Standard:

(C99, 7.20.3p1) "If the size of the space requested is zero, the behavior is implementation defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object."

Answer 2

Why does malloc(0) actually return a valid pointer for writing?

It doesn't return a valid pointer for writing. It returns a valid pointer for not using it. Or it may return NULL as well since the C standard specifies this case to be implementation defined.

Answer 3

malloc() is supposed to return a void* pointer. And it faithfully does that. But leads to UB when you dereference it.