The C subroutine library depends on a handful of subroutine calls for operating system services. If you use the C library on a system that complies with the POSIX.1 standard (also known as IEEE 1003.1), most of these subroutines are supplied with your operating system.
If some of these subroutines are not provided with your system--in
the extreme case, if you are developing software for a "bare board"
system, without an OS--you will at least need to provide do-nothing
stubs (or subroutines with minimal functionality) to allow your
programs to link with the subroutines in libc.a.