System calls

There are dozens of useful procedures available in standard libraries, including procedures for

....Mathematical functions (sin, cos)
....Random numbers (rand, random)
....Sorting (qsort)
....String functions (strcpy, strcat)
....Input and output (gets, fseek, fopen)
....OS information (getpid, getrusage)
....Interprocess communication (socket, send, recv)
....File system information (stat, link, unlink)
....Multitasking (fork, wait)
....Directories (opendir, readdir)
....Parsing command lines (getopt)
....}

The online manual also explains these.