(you duped your comment under the other subthread)

From C89, §7.10.3 "Memory management functions":

> If the size of the space requested is > zero, the behavior is implementation-defined; the value returned shall be either a null pointer or a > unique pointer.

The wording is different for C99 and POSIX, but I went back as far as possible (despite the poor source material; unlike later standards C89 is only accessible in scans and bad OCR, and also has catastrophic numbering differences). K&R C specifies nothing (it's often quite useless; people didn't actually write against K&R C but against the common subset of extensions of platforms they cared about), but its example implementation adds a block header without checking for 0 so it ends up doing the "unique non-NULL pointer" thing.