— Function: int kill (pid_t pid, int signum)

The kill function sends the signal signum to the process or process group specified by pid. Besides the signals listed in Standard Signals, signum can also have a value of zero to check the validity of the pid.

The pid specifies the process or process group to receive the signal:

pid > 0

The process whose identifier is pid.

pid == 0

All processes in the same process group as the sender.

pid < -1

The process group whose identifier is −pid.

pid == -1

If the process is privileged, send the signal to all processes except for some special system processes. Otherwise, send the signal to all processes with the same effective user ID.

A process can send a signal to itself with a call like kill (getpid(), signum). If kill is used by a process to send a signal to itself, and the signal is not blocked, then kill delivers at least one signal (which might be some other pending unblocked signal instead of the signal signum) to that process before it returns.

The return value from kill is zero if the signal can be sent successfully. Otherwise, no signal is sent, and a value of -1 is returned. If pid specifies sending a signal to several processes, kill succeeds if it can send the signal to at least one of them. There's no way you can tell which of the processes got the signal or whether all of them did.

The following errno error conditions are defined for this function:

EINVAL

The signum argument is an invalid or unsupported number.

EPERM

You do not have the privilege to send a signal to the process or any of the processes in the process group named by pid.

ESCRH

The pid argument does not refer to an existing process or group.

— Function: int killpg (int pgid, int signum)

This is similar to kill, but sends signal signum to the process group pgid. This function is provided for compatibility with BSD; using kill to do this is more portable.