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

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

       sigqueue — queue a signal to a process

SYNOPSIS         top

       #include <signal.h>

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

DESCRIPTION         top

       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         top

       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         top

       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         top

       None.

APPLICATION USAGE         top

       None.

RATIONALE         top

       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.1‐2008 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.1‐2008 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         top

       None.

SEE ALSO         top

       Section 2.8.1, Realtime Signals

       The Base Definitions volume of POSIX.1‐2008, signal.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                        SIGQUEUE(3P)