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



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

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(3)
