Using integer codes like 0 and -1 were the old style of returning errors. Often combined with some undocumented features.

It is really wrong, but that is caused by the C-functions returning only one integer of data. And the bad C libraries. Null used to be -1 in some compilers. Mixing pointers as integers is a recipe for disaster, not compatible with some CPU-s, but it is C-standard.

In the windows32 API they improved it a bit, with functions that only returned an Error code. They required struct addresses in which results were placed. I think that the process structure is also a lot better than the fork() function.