The kill() system call can be used to send any signal to any process group or process.
If pid is positive, then signal sig is sent to pid.
If pid equals 0, then sig is sent to every process in the process group of the current process.
If pid equals -1, then sig is sent to every process for which the calling process has permission to send signals.
If pid is less than -1, then sig is sent to every process in the process group -pid.
Return 0 for success, -1 for error.

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