Let's not forget the system call! (jesus, I have to stop noding C functions...)
    int kill( pid_t pid, int sig );
pid_t is defined in sys/types.h, and the function is prototyped in signal.h.

The pid arg can have special meaning. If it's zero, the current process and all children are sent the signal. If it's -1, all processes are sent it, except pid 1. Otherwise, it specifies a pid.

As for the second arg, see unix signals.