kill

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 pro-
       cesses 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 desig-
       nated 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 unspeci-
       fied set of system processes) whose process group ID is  equal  to  the
       process  group  ID of the sender, and for which the process has permis-
       sion to send a signal.

       If pid is -1, sig shall be sent to all processes (excluding an unspeci-
       fied  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  send-
       ing 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 exis-
       tence 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 sig-
       nal shall be sent.
              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  Sys-
       tem V and most other implementations, such as Version 7 or 4.3 BSD. The
       semantics chosen for this volume  of  IEEE Std 1003.1-2001  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 other-
       wise 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 IEEE Std 1003.1-2001, although a  conforming
       implementation could provide such an extension.

       The  implementation-defined  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
       IEEE Std 1003.1-2001, 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 neces-
       sarily when  signal()  is  used.  The  developers  of  this  volume  of
       IEEE Std 1003.1-2001  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().
       nals  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 pre-
       vent  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.

       Existing implementations vary on the result of a kill() with pid  indi-
       cating  an  inactive  process  (a  terminated process that has not been
       waited for by its parent). Some indicate success on such a  call  (sub-
       ject  to  permission  checking), while others give an error of [ESRCH].
       Since  the  definition  of  process  lifetime   in   this   volume   of
       IEEE Std 1003.1-2001  covers  inactive  processes, the [ESRCH] error as
       described is inappropriate in this case. In particular, this means that
       an  application cannot have a parent process check for termination of a
       particular child with kill(). (Usually this is done with the null  sig-
       nal; this can be done reliably with waitpid().)

       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() , the Base
       Definitions volume of IEEE Std 1003.1-2001, <signal.h>, <sys/types.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 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 .



IEEE/The Open Group                  2003                              KILL(P)