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SIGQUEUE(3P) POSIX Programmer's Manual SIGQUEUE(3P)
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.
sigqueue — queue a signal to a process
#include <signal.h>
int sigqueue(pid_t pid, int signo, const union sigval value);
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.
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.
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.
None.
None.
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.
None.
Section 2.8.1, Realtime Signals
The Base Definitions volume of POSIX.1‐2008, signal.h(0p)
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)
Pages that refer to this page: signal.h(0p), kill(3p), pthread_sigmask(3p)