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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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, 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.1‐2017 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.
None.
Section 2.8.1, Realtime Signals
The Base Definitions volume of POSIX.1‐2017, signal.h(0p)
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(3P)
Pages that refer to this page: signal.h(0p), kill(3p), pthread_sigmask(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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