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). Providing such stubs allows your programs to link with the subroutines in libc.a. 0