NAME

       kill - send signal to a process

SYNOPSIS

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

       int kill(pid_t pid, int sig);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       kill(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

DESCRIPTION

       The kill() 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 the process with the ID specified by pid.

       If  pid  equals  0,  then sig is sent to every process in the process group of the calling
       process.

       If pid equals -1, then sig is sent to every process for  which  the  calling  process  has
       permission to send signals, except for process 1 (init), but see below.

       If pid is less than -1, then sig is sent to every process in the process group whose ID is
       -pid.

       If sig is 0, then no signal is sent, but error checking is still performed;  this  can  be
       used to check for the existence of a process ID or process group ID.

       For  a  process  to  have  permission to send a signal it must either be privileged (under
       Linux: have the CAP_KILL capability), or the real or effective  user  ID  of  the  sending
       process  must  equal  the real or saved set-user-ID of the target process.  In the case of
       SIGCONT it suffices when the sending and receiving processes belong to the  same  session.
       (Historically, the rules were different; see NOTES.)

RETURN VALUE

       On  success  (at  least one signal was sent), zero is returned.  On error, -1 is returned,
       and errno is set appropriately.

ERRORS

       EINVAL An invalid signal was specified.

       EPERM  The process does not have permission to send  the  signal  to  any  of  the  target
              processes.

       ESRCH  The  pid or process group does not exist.  Note that an existing process might be a
              zombie, a process which  already  committed  termination,  but  has  not  yet  been
              wait(2)ed for.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES

       The  only  signals that can be sent to process ID 1, the init process, are those for which
       init has explicitly installed signal handlers.  This is done to assure the system  is  not
       brought down accidentally.

       POSIX.1  requires that kill(-1,sig) send sig to all processes that the calling process may
       send signals to, except possibly for some implementation-defined system processes.   Linux
       allows  a process to signal itself, but on Linux the call kill(-1,sig) does not signal the
       calling process.

       POSIX.1 requires that if a process sends a signal to itself, and the sending  thread  does
       not  have the signal blocked, and no other thread has it unblocked or is waiting for it in
       sigwait(3), at least one unblocked signal must be delivered to the sending  thread  before
       the kill() returns.

   Linux notes
       Across  different  kernel versions, Linux has enforced different rules for the permissions
       required for an unprivileged process to send a signal to another process.  In kernels  1.0
       to  1.2.2, a signal could be sent if the effective user ID of the sender matched effective
       user ID of the target, or the real user ID of the sender matched the real user ID  of  the
       target.   From  kernel 1.2.3 until 1.3.77, a signal could be sent if the effective user ID
       of the sender matched either the real or effective user ID of  the  target.   The  current
       rules, which conform to POSIX.1, were adopted in kernel 1.3.78.

BUGS

       In  2.6  kernels  up  to and including 2.6.7, there was a bug that meant that when sending
       signals to a process group, kill() failed with the error EPERM if the caller did not  have
       permission  to  send  the  signal  to  any (rather than all) of the members of the process
       group.  Notwithstanding this error return, the signal was still delivered to  all  of  the
       processes for which the caller had permission to signal.

SEE ALSO

       _exit(2),   killpg(2),   signal(2),   tkill(2),   exit(3),  sigqueue(3),  capabilities(7),
       credentials(7), signal(7)

COLOPHON

       This page is part of release 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.