man_2_kill.txt

KILL(2)                            Linux Programmer's Manual                           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 per‐
       formed; 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 dif‐
       ferent; 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  calling  process  does not have permission to send the signal to any of the
              target processes.

       ESRCH  The target 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  permis‐
       sions  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),  pidfd_send_signal(2),  signal(2),  tkill(2),  exit(3),  killpg(3),
       sigqueue(3), capabilities(7), credentials(7), signal(7)

COLOPHON
       This page is part of release 5.05 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/.

Linux                                     2019-10-10                                   KILL(2)