Complex sentence: POSIX (and SUS) specifies a number of routines that should be available over and above those in the C standard library proper; these are often implemented alongside the C standard library functionality, with varying degrees of closeness.
Split into simpler sentences: POSIX (and SUS) specifies a number of routines that should be available over and above those in the C standard library proper. These are often implemented alongside the C standard library functionality, with varying degrees of closeness.