The kill(2) 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 except for the first one.
If pid is less than -1, then sig is sent to every process in the process group -pid.
If sig is 0, then no signal is sent, but error checking is still performed. This is an excellent way to see if a process is still running, send it signal 0, then check to see if you got ESRCH as an error.
It is impossible to send a signal to task number one, the init process, for which it has not installed a signal handler. This is done to assure the system is not brought down accidentally.
SVr4, SVID, POSIX.1, X/OPEN, BSD 4.3
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