PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

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

SYNOPSIS         top

       #include <signal.h>

       int kill(pid_t pid, int sig);

DESCRIPTION         top

       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         top

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

ERRORS         top

       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         top

       None.

APPLICATION USAGE         top

       None.

RATIONALE         top

       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.1‐2008 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.1‐2008, 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.1‐2008, 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.

       Existing implementations vary on the result of a kill() with pid
       indicating an inactive process (a terminated process that has not
       been waited for by its parent). Some indicate success on such a call
       (subject to permission checking), while others give an error of
       [ESRCH].  Since the definition of process lifetime in this volume of
       POSIX.1‐2008 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 signal; 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         top

       None.

SEE ALSO         top

       getpid(3p), raise(3p), setsid(3p), sigaction(3p), sigqueue(3p),
       wait(3p)

       The Base Definitions volume of POSIX.1‐2008, signal.h(0p),
       sys_types.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                            KILL(3P)

Pages that refer to this page: signal.h(0p)kill(1p)abort(3p)getpgrp(3p)getpid(3p)getppid(3p)killpg(3p)posix_spawn(3p)pthread_kill(3p)pthread_sigmask(3p)raise(3p)setpgrp(3p)sigaction(3p)