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@(#)kill.2 6.1 (Berkeley) 05/09/85
int pid, sig;
The sending and receiving processes must have the same effective user ID, otherwise this call is restricted to the super-user. A single exception is the signal SIGCONT which may always be sent to any child or grandchild of the current process.
If the process number is 0, the signal is sent to all other processes in the sender's process group; this is a variant of killpg (2).
If the process number is -1, and the user is the super-user, the signal is broadcast universally except to system processes and the process sending the signal.
Processes may send signals to themselves.
15 [EINVAL] Sig is not a valid signal number.
15 [ESRCH] No process can be found corresponding to that specified by pid.
15 [EPERM] The sending process is not the super-user and its effective user id does not match the effective user-id of the receiving process.