NAME

kill − send a signal to a process or a group of processes

SYNOPSIS

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

int kill (pid_t pid, int sig);

DESCRIPTION

kill sends a signal to a process or a group of processes.  The process or group of processes to which the signal is to be sent is specified by pid. The signal that is to be sent is specified by sig and is either one from the list given in signal [see signal(5)], or 0.  If sig is 0 (the null signal), error checking is performed but no signal is actually sent.  This can be used to check the validity of pid.

The real or effective user ID of the sending process must match the real or saved [from exec(2)] user ID of the receiving process unless the effective user ID of the sending process is superuser, [see intro(2)], or sig is SIGCONT and the sending process has the same session ID as the receiving process. 

The process with ID 0 and the process with ID 1 are special processes [see intro(2)] and will be referred to below as proc0 and proc1, respectively. 

If pid is greater than 0, sig will be sent to the process whose process ID is equal to pid. pid may equal 1. 

If pid is negative but not (pid_t)−1, sig will be sent to all processes whose process group ID is equal to the absolute value of pid and for which the process has permission to send a signal. 

If pid is 0, sig will be sent to all processes excluding proc0 and proc1 whose process group ID is equal to the process group ID of the sender.  Permission is needed to send a signal to process groups. 

If pid is (pid_t)−1 and the effective user ID of the sender is not superuser, sig will be sent to all processes excluding proc0 and proc1 whose real user ID is equal to the effective user ID of the sender. 

If pid is (pid_t)−1 and the effective user ID of the sender is superuser, sig will be sent to all processes excluding proc0 and proc1. 

kill will fail and no signal will be sent if one or more of the following are true:

EINVAL sig is not a valid signal number. 

EINVAL sig is SIGKILL and pid is (pid_t)1 (that is, pid specifies proc1). 

ESRCH No process or process group can be found corresponding to that specified by pid.

EPERM The user ID of the sending process is not privileged, and its real or effective user ID does not match the real or saved user ID of the receiving process, and the calling process is not sending SIGCONT to a process that shares the same session ID. 

SEE ALSO

kill(1), getpid(2), getsid(2), intro(2), setpgrp(2), sigaction(2), signal(2), sigsend(2). 

NOTES

sigsend is a more versatile way to send signals to processes.  The user is encouraged to use sigsend instead of kill. 

DIAGNOSTICS

Upon successful completion, a value of 0 is returned.  Otherwise, a value of −1 is returned and errno is set to indicate the error.