Different OS provides different APIs. Operating system designers are free to choose alternative procedure names or parameters as long as they offer equivalent functionality. System calls To a programmer, system calls look and act like function calls .
Many system designers considers it unwise to create new system calls unless necessary.
When a process opens a file, the system places a pointer to the internal data structures for that file in the process' file descriptor table and returns the table index to the caller. UNIX allocates socket descriptors in the same descriptor table as file descriptors. Although the internal data structure for a socket contains many fields, the system leaves most of them unfilled when it creates the socket. The application has to make additional system call to fill in information in the socket data structure before the socket can be used. It never needs to use the sockaddr structure.