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/.

Linux                                              2015-08-08                                            KILL(2)