System call vs Function call

Question

What is the difference between a system call and a function call? Is fopen() a system call or a function call?

Answer 1

fopen is a function call.

A system call interacts with the underlying OS, which manages resources. Its orders of magnitud more expensive than a function call, because many steps have to be taken to preserve the state of the process that made the syscall.

On *nix systems, fopen wraps open, which makes the system call (open is the C - wrapper for the syscall). The same happens with fread /read, fwrite / write , etc..

Here there's a nice description of the tasks executed by a unix syscall.