This is C, not POSIX. POSIX defers to ISO C but has extensions and more strictly defined behavior/return values. Example (first function I randomly looked at)

This is C, not POSIX. POSIX defers to ISO C but has extensions and more strictly defined behavior/return values. Example (first function I randomly looked at) remove() - PDCLib won't remove directories or currently fopened files. ISO C doesn't mention directory support and removing an open file is implementation defined. POSIX says it is equivalent to unlink() or removdir() as appropriate.