[magicalhippo]

But the letter is non-specific. It doesn't clarify if unique refers to unique when compared to non-zero allocations, or unique when called multiple times.

The C99 standard[1] seems to have worded it more precisely:

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.

[1]: https://rgambord.github.io/c99-doc/sections/7/20/3/index.htm...

[f1shy]

I think that is the problem. I understood unique as „only one“ which means always returns the same. It is imho not clear enough.

[bobmcnamara]

It even replaced unique with "disjoint from any other object".

-1 seems to be disjoint from all objects.

[magicalhippo]

Sure, but always returning -1 would not satisfy "the behavior is as if the size were some nonzero value", as that would mean two consecutive calls to malloc(0) would return the same value. Which is not how malloc() with a non-zero size behaves per the previous part of the definition.

Writing a specification is hard...