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KILL(3P)                   POSIX Programmer's Manual                  KILL(3P)

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 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.
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  sig-
              nal 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 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().
       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 sig-
       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  Defini-
       tions 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(3P)