sigqueue(3p) — Linux manual page


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, 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()

       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




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

       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.



SEE ALSO         top

       Section 2.8.1, Realtime Signals

       The Base Definitions volume of POSIX.1‐2017, signal.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                      SIGQUEUE(3P)

Pages that refer to this page: signal.h(0p)kill(3p)pthread_sigmask(3p)