KILL(2)                    Linux Programmer's Manual                   KILL(2)

       kill - send signal to a process

       #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

       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 call-
       ing  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

       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 name-
       space 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  re-
       ceiving processes belong to the same session.  (Historically, the rules
       were different; see NOTES.)

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

       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 termi-
              nated execution, but has not yet been wait(2)ed for.

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

       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 implemen-
       tation-defined system processes.  Linux allows a process to signal  it-
       self,  but  on  Linux the call kill(-1,sig) does not signal the calling

       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  un-
       blocked  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 sig-
       nal 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.

       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.  Notwith-
       standing this error return, the signal was still delivered  to  all  of
       the processes for which the caller had permission to signal.

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

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Linux                             2019-10-10                           KILL(2)
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