focal (2) kill.2.gz

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 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 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),  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/.