NAME

     kill — send signal to a process

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

     int
     kill(pid_t pid, int sig);

DESCRIPTION

     The kill() system call sends the signal given by sig to pid, a process or a group of
     processes.  The sig argument 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 user
     must be the super-user, or the real or saved user ID of the receiving process must match the
     real or effective user ID of the sending process.  A single exception is the signal SIGCONT,
     which may always be sent to any process with the same session ID as the sender.  In
     addition, if the security.bsd.conservative_signals sysctl(9) is set to 1, the user is not a
     super-user, and the receiver is set-uid, then only job control and terminal control signals
     may be sent (in particular, only SIGKILL, SIGINT, SIGTERM, SIGALRM, SIGSTOP, SIGTTIN,
     SIGTTOU, SIGTSTP, SIGHUP, SIGUSR1, SIGUSR2).

     If pid is greater than zero:
             The sig signal is sent to the process whose ID is equal to pid.

     If pid is zero:
             The sig signal 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(2).

     If pid is -1:
             If the user has super-user privileges, the signal is sent to all processes excluding
             system processes (with P_SYSTEM flag set), process with ID 1 (usually init(8)), and
             the process sending the signal.  If the user is not the super user, the signal is
             sent to all processes which the caller has permissions to, excluding the process
             sending the signal.  No error is returned if any process could be signaled.

     If the process number is negative but not -1, the signal is sent to all processes whose
     process group ID is equal to the absolute value of the process number.  This is a variant of
     killpg(2).

RETURN VALUES

     The kill() function returns the value 0 if successful; otherwise the value -1 is returned
     and the global variable errno is set to indicate the error.

ERRORS

     The kill() system call will fail and no signal will be sent if:

     [EINVAL]           The sig argument is not a valid signal number.

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

     [EPERM]            The sending process does not have permission to send sig to any receiving
                        process.

SEE ALSO

     getpgrp(2), getpid(2), killpg(2), sigaction(2), sigqueue(2), raise(3), init(8)

STANDARDS

     The kill() system call is expected to conform to ISO/IEC 9945-1:1990 (“POSIX.1”).

HISTORY

     A version of the kill() function appeared in Version 3 AT&T UNIX.  The signal number was
     added to the kill() function in Version 4 AT&T UNIX.