SIGQUEUE(3)                POSIX Programmer's Manual               SIGQUEUE(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
       sigqueue -- queue a signal to a process

SYNOPSIS
       #include <signal.h>

       int sigqueue(pid_t pid, int signo, union sigval value);

DESCRIPTION
       The sigqueue() function shall cause the signal specified by signo to be
       sent with the value specified by value to the process specified by pid.
       If signo is zero (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.

       The conditions required for a process to have permission to queue a
       signal to another process are the same as for the kill() function.

       The sigqueue() function shall return immediately. If SA_SIGINFO is set
       for signo and if the resources were available to queue the signal, the
       signal shall be queued and sent to the receiving process. If SA_SIGINFO
       is not set for signo, then signo shall be sent at least once to the
       receiving process; it is unspecified whether value shall be sent to the
       receiving process as a result of this call.

       If the value of pid causes signo to be generated for the sending
       process, and if signo is not blocked for the calling thread and if no
       other thread has signo unblocked or is waiting in a sigwait() function
       for signo, either signo or at least the pending, unblocked signal shall
       be delivered to the calling thread before the sigqueue() function
       returns. Should any multiple pending signals in the range SIGRTMIN to
       SIGRTMAX be selected for delivery, it shall be the lowest numbered one.
       The selection order between realtime and non-realtime signals, or
       between multiple pending non-realtime signals, is unspecified.

RETURN VALUE
       Upon successful completion, the specified signal shall have been
       queued, and the sigqueue() function shall return a value of zero.
       Otherwise, the function shall return a value of -1 and set errno to
       indicate the error.

ERRORS
       The sigqueue() function shall fail if:

       EAGAIN No resources are available to queue the signal. The process has
              already queued {SIGQUEUE_MAX} signals that are still pending at
              the receiver(s), or a system-wide resource limit has been
              exceeded.

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

       EPERM  The process does not have appropriate privileges to send the
              signal to the receiving process.

       ESRCH  The process pid does not exist.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       The sigqueue() function allows an application to queue a realtime
       signal to itself or to another process, specifying the application-
       defined value. This is common practice in realtime applications on
       existing realtime systems.  It was felt that specifying another
       function in the sig...  name space already carved out for signals was
       preferable to extending the interface to kill().

       Such a function became necessary when the put/get event function of the
       message queues was removed. It should be noted that the sigqueue()
       function implies reduced performance in a security-conscious
       implementation as the access permissions between the sender and
       receiver have to be checked on each send when the pid is resolved into
       a target process. Such access checks were necessary only at message
       queue open in the previous interface.

       The standard developers required that sigqueue() have the same
       semantics with respect to the null signal as kill(), and that the same
       permission checking be used. But because of the difficulty of
       implementing the ``broadcast'' semantic of kill() (for example, to
       process groups) and the interaction with resource allocation, this
       semantic was not adopted. The sigqueue() function queues a signal to a
       single process specified by the pid argument.

       The sigqueue() function can fail if the system has insufficient
       resources to queue the signal. An explicit limit on the number of
       queued signals that a process could send was introduced. While the
       limit is ``per-sender'', this volume of POSIX.1-2017 does not specify
       that the resources be part of the state of the sender. This would
       require either that the sender be maintained after exit until all
       signals that it had sent to other processes were handled or that all
       such signals that had not yet been acted upon be removed from the
       queue(s) of the receivers. This volume of POSIX.1-2017 does not
       preclude this behavior, but an implementation that allocated queuing
       resources from a system-wide pool (with per-sender limits) and that
       leaves queued signals pending after the sender exits is also permitted.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.8.1, Realtime Signals

       The Base Definitions volume of POSIX.1-2017, <signal.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                          SIGQUEUE(3)
