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KILL(2) System Calls Manual KILL(2)


killsend signal to a process


#include <signal.h>
kill(pid_t pid, int sig);


The kill() function sends the signal given by sig to pid, a process or a group of processes. sig may be one of the signals specified in sigaction(2) or it may be 0, in which case error checking is performed but no signal is actually sent. This can be used to check the validity of pid.
For a process to have permission to send a signal to a process designated by pid, the real or effective user ID of the receiving process must match that of the sending process or the user must have appropriate privileges (such as given by a set-user-ID program or the user is the superuser). A single exception is the signal SIGCONT, which may always be sent to any process with the same session ID as the caller.
If pid is greater than zero:
sig is sent to the process whose ID is equal to pid.
If pid is zero:
sig is sent to all processes whose group ID is equal to the process group ID of the sender, and for which the process has permission; this is a variant of killpg(3).
If pid is -1:
If the user has superuser privileges, the signal is sent to all processes excluding system processes and the process sending the signal. If the user is not the superuser, the signal is sent to all processes with the same uid as the user excluding the process sending the signal. No error is returned if any process could be signaled.
If pid is negative but not -1:
sig is sent to all processes whose process group ID is equal to the absolute value of pid; this is a variant of killpg(3).
If the value of pid causes sig to be sent to the calling process, either sig or at least one pending unblocked signal will be delivered before kill() returns unless sig is blocked in the calling thread, sig is unblocked in another thread, or another thread is waiting for sig in sigwait().
Setuid and setgid processes are dealt with slightly differently. For the non-root user, to prevent attacks against such processes, some signal deliveries are not permitted and return the error EPERM. The following signals are allowed through to this class of processes: SIGKILL, SIGINT, SIGTERM, SIGSTOP, SIGTTIN, SIGTTOU, SIGTSTP, SIGHUP, SIGUSR1, SIGUSR2.


Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


kill() will fail and no signal will be sent if:
sig is not a valid signal number.
No process can be found corresponding to that specified by pid.
The sending process is not the superuser and its effective user ID does not match the effective user ID of the receiving process. When signaling a process group, this error is returned if none of the members of the group could be signaled.


getpgrp(2), getpid(2), sigaction(2), killpg(3), raise(3)


The kill() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).


The kill() system call first appeared in Version 3 AT&T UNIX. The sig argument was introduced in Version 4 AT&T UNIX.
May 6, 2017 OpenBSD-current