SIGWAIT
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
PROLOG
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
sigwait
--- wait for queued signals
SYNOPSIS
#include <signal.h>
int sigwait(const sigset_t *restrict set, int *restrict sig);
DESCRIPTION
The
sigwait()
function shall select a pending signal from
set,
atomically clear it from the system's set of pending signals, and
return that signal number in the location referenced by
sig.
If prior to the call to
sigwait()
there are multiple pending instances of a single signal number, it is
implementation-defined whether upon successful return there are any
remaining pending signals for that signal number.
If the implementation supports queued signals and there are multiple
signals queued for the signal number selected, the first such queued
signal shall cause a return from
sigwait()
and the remainder shall remain queued. If no signal in
set
is pending at the time of the call, the thread shall be suspended
until one or more becomes pending. The signals defined by
set
shall have been blocked at the time of the call to
sigwait();
otherwise, the behavior is undefined. The effect of
sigwait()
on the signal actions for the signals in
set
is unspecified.
If more than one thread is using
sigwait()
to wait for the same signal, no more than one of these threads shall
return from
sigwait()
with the signal number. If more than a single thread is blocked in
sigwait()
for a signal when that signal is generated for the process, it is
unspecified which of the waiting threads returns from
sigwait().
If the signal is generated for a specific thread, as by
pthread_kill(),
only that thread shall return.
Should any of the multiple pending signals in the range SIGRTMIN to
SIGRTMAX be selected, 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,
sigwait()
shall store the signal number of the received signal at the location
referenced by
sig
and return zero. Otherwise, an error number shall be returned to
indicate the error.
ERRORS
The
sigwait()
function may fail if:
- EINVAL
-
The
set
argument contains an invalid or unsupported signal number.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
To provide a convenient way for a thread to wait for a signal, this volume of POSIX.1-2017
provides the
sigwait()
function. For most cases where a thread has to wait for a signal, the
sigwait()
function should be quite convenient, efficient, and adequate.
However, requests were made for a lower-level primitive than
sigwait()
and for semaphores that could be used by threads. After some
consideration, threads were allowed to use semaphores and
sem_post()
was defined to be async-signal-safe.
In summary, when it is necessary for code run in response to an
asynchronous signal to notify a thread,
sigwait()
should be used to handle the signal. Alternatively, if the
implementation provides semaphores, they also can be used, either
following
sigwait()
or from within a signal handling routine previously registered with
sigaction().
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.4,
Signal Concepts,
Section 2.8.1,
Realtime Signals,
pause(),
pthread_sigmask(),
sigaction(),
sigpending(),
sigsuspend(),
sigtimedwait()
The Base Definitions volume of POSIX.1-2017,
<signal.h>,
<time.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 .