Are Unix/Linux system calls part of POSIX library functions?

Question

Are Unix/Linux system calls all or mostly in POSIX?

Many Linux/Unix programming books say that POSIX library functions may be wrappers of OS system calls, or may be not. E.g. http://www.makelinux.net/books/lkd2/ch05lev1sec1, and https://www.safaribooksonline.com/library/view/understanding-the-linux/0596005652/ch10s01.html

A part (called the Single UNIX Specification) of POSIX defines UNIX. Therefore I think POSIX defines the system calls of Unix (and of Linux).

Then are Unix/Linux system calls part of POSIX library functions?

Thanks.

Answer 1

Linux system calls are listed in syscalls(2). Most are POSIX, but some are specific to Linux (e.g. signalfd(2), etc...). POSIX is a specification and does not "know" about syscalls (which, in the POSIX view, are an implementation detail).

Some functions are standardized in POSIX but implemented in Linux library code, e.g. dlopen(3) (see POSIX dlopen) built above mmap(2)...