PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

NAME

       kill — send a signal to a process or a group of processes

SYNOPSIS

       #include <signal.h>

       int kill(pid_t pid, int sig);

DESCRIPTION

       The  kill()  function  shall  send  a  signal to a process or a group of processes specified by pid.  The
       signal to be sent is specified by sig and is either one from the list given in <signal.h> or 0. If sig is
       0 (the null signal), error checking is performed but no signal is actually sent. The null signal  can  be
       used to check the validity of pid.

       For  a  process  to  have  permission to send a signal to a process designated by pid, unless the sending
       process has appropriate privileges, the real or effective user ID of the sending process shall match  the
       real or saved set-user-ID of the receiving process.

       If pid is greater than 0, sig shall be sent to the process whose process ID is equal to pid.

       If  pid is 0, sig shall be sent to all processes (excluding an unspecified set of system processes) whose
       process group ID is equal to the process group ID of the sender, and for which the process has permission
       to send a signal.

       If pid is -1, sig shall be sent to all processes (excluding an unspecified set of system  processes)  for
       which the process has permission to send that signal.

       If  pid  is  negative,  but  not  -1, sig shall be sent to all processes (excluding an unspecified set of
       system processes) whose process group ID is equal to the absolute value of pid, and for which the process
       has permission to send a signal.

       If the value of pid causes sig to be generated for the sending process, and if sig is not blocked for the
       calling thread and if no other thread has sig unblocked or is waiting in a sigwait()  function  for  sig,
       either  sig  or  at  least  one  pending unblocked signal shall be delivered to the sending thread before
       kill() returns.

       The user ID tests described above shall not be applied when sending SIGCONT to a process that is a member
       of the same session as the sending process.

       An implementation that provides extended security  controls  may  impose  further  implementation-defined
       restrictions on the sending of signals, including the null signal. In particular, the system may deny the
       existence of some or all of the processes specified by pid.

       The  kill()  function  is  successful  if  the process has permission to send sig to any of the processes
       specified by pid.  If kill() fails, no signal shall be sent.

RETURN VALUE

       Upon successful completion, 0 shall be returned. Otherwise,  -1  shall  be  returned  and  errno  set  to
       indicate the error.

ERRORS

       The kill() function shall fail if:

       EINVAL The value of the sig argument is an invalid or unsupported signal number.

       EPERM  The process does not have permission to send the signal to any receiving process.

       ESRCH  No process or process group can be found corresponding to that specified by pid.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       The   semantics   for   permission  checking  for  kill()  differed  between  System  V  and  most  other
       implementations, such as Version 7 or 4.3 BSD. The semantics chosen for this volume of POSIX.1‐2017 agree
       with System V.  Specifically, a set-user-ID process cannot protect itself against signals  (or  at  least
       not  against  SIGKILL)  unless  it  changes  its real user ID.  This choice allows the user who starts an
       application to send it signals even if it changes its effective user ID.  The other semantics  give  more
       power to an application that wants to protect itself from the user who ran it.

       Some implementations provide semantic extensions to the kill() function when the absolute value of pid is
       greater than some maximum, or otherwise special, value. Negative values are a flag to kill().  Since most
       implementations  return  [ESRCH]  in  this  case,  this  behavior  is  not  included  in  this  volume of
       POSIX.1‐2017, although a conforming implementation could provide such an extension.

       The unspecified processes to which a signal cannot be sent may include the scheduler or init.

       There was initially strong sentiment to specify that, if pid specifies that  a  signal  be  sent  to  the
       calling  process  and  that  signal is not blocked, that signal would be delivered before kill() returns.
       This would permit a process to call kill() and  be  guaranteed  that  the  call  never  return.  However,
       historical implementations that provide only the signal() function make only the weaker guarantee in this
       volume  of  POSIX.1‐2017,  because  they  only  deliver one signal each time a process enters the kernel.
       Modifications to such implementations to support the sigaction() function generally require entry to  the
       kernel  following  return  from  a  signal-catching  function,  in order to restore the signal mask. Such
       modifications have the effect of satisfying the stronger requirement, at least when sigaction() is  used,
       but  not  necessarily  when  signal()  is  used.  The  standard developers considered making the stronger
       requirement except when signal() is used, but felt this would be unnecessarily complex. Implementors  are
       encouraged to meet the stronger requirement whenever possible. In practice, the weaker requirement is the
       same,  except  in  the  rare case when two signals arrive during a very short window. This reasoning also
       applies to a similar requirement for sigprocmask().

       In 4.2 BSD, the SIGCONT signal can be sent to any  descendant  process  regardless  of  user-ID  security
       checks.   This  allows  a  job  control shell to continue a job even if processes in the job have altered
       their user IDs (as in the su command). In keeping with the addition of the concept of  sessions,  similar
       functionality  is  provided  by allowing the SIGCONT signal to be sent to any process in the same session
       regardless of user ID security checks. This is less restrictive than  BSD  in  the  sense  that  ancestor
       processes  (in  the same session) can now be the recipient.  It is more restrictive than BSD in the sense
       that descendant processes that form new sessions are now  subject  to  the  user  ID  checks.  A  similar
       relaxation  of  security  is  not  necessary  for  the  other job control signals since those signals are
       typically sent by the terminal driver in recognition of special  characters  being  typed;  the  terminal
       driver bypasses all security checks.

       In  secure  implementations,  a  process  may  be  restricted from sending a signal to a process having a
       different security label.  In order to prevent the existence or nonexistence of a process from being used
       as a covert channel, such processes should appear nonexistent to the sender; that is, [ESRCH]  should  be
       returned, rather than [EPERM], if pid refers only to such processes.

       Historical  implementations  varied  on the result of a kill() with pid indicating a zombie process. Some
       indicated success on such a call (subject to permission checking), while others gave an error of [ESRCH].
       Since the definition of process lifetime in this volume of  POSIX.1‐2017  covers  zombie  processes,  the
       [ESRCH]  error as described is inappropriate in this case and implementations that give this error do not
       conform. This means that an application  cannot  have  a  parent  process  check  for  termination  of  a
       particular child by sending it the null signal with kill(), but must instead use waitpid() or waitid().

       There  is  some  belief  that the name kill() is misleading, since the function is not always intended to
       cause process termination. However, the name is common to all historical implementations, and any  change
       would be in conflict with the goal of minimal changes to existing application code.

FUTURE DIRECTIONS

       None.

SEE ALSO

       getpid(), raise(), setsid(), sigaction(), sigqueue(), wait()

       The Base Definitions volume of POSIX.1‐2017, <signal.h>, <sys_types.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical  and  Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE  and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                          KILL(3POSIX)