Most of the signals considered so far have been generated either automatically by a process changing state, by a hardware error like a floating point exception, or they have been generated by user intervention at the keyboard with interrupt and quit signals. In addition to this, it is also possible for a process to send any signal to another process deliberately if it has appropriate permissions. This is done by using the kill() system call:

	#include <signal.h>
	#include <sys/types.h>

	int kill(pid_t pid, int sig);

The sig parameter specifies which signal to send and the pid parameter specifies where to send it. The various values of pid have the following meanings:

pid > 0

This will send signal sig to the process whose process ID is pid.

pid = 0

Sends signal sig to all processes with the same process group ID as the calling process.

pid = -1

Sends signal sig to all processes in the system except the init process and the calling process. (This is Linux specific.)

pid < -1

Sends signal sig to all the processes in the process group whose process group ID matches the absolute value of pid (i.e. to process group -pid).

In order for kill() to send a signal, the effective user ID of the calling process must be root, or it must match either the real or effective user ID of the receiving process.

Next: pause System Call

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