kill(2)

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

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 existence and permission
   checks are still performed; this can be used to check for the existence
   of  a  process  ID  or process group ID that the caller is permitted to
   signal.

   For a process to have permission to send a signal, it  must  either  be
   privileged  (under  Linux:  have  the  CAP_KILL  capability in the user
   namespace of the target process), 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 process or process group  does  not  exist.   Note  that  an
          existing   process  might  be  a  zombie,  a  process  that  has
          terminated execution, 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

   kill(1),    _exit(2),    signal(2),   tkill(2),   exit(3),   killpg(3),
   sigqueue(3), capabilities(7), credentials(7), signal(7)

COLOPHON

   This page is part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.

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