Section: POSIX Programmer's Manual (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.
--- wait for queued signals
int sigwait(const sigset_t *restrict set, int *restrict sig);
function shall select a pending signal from
atomically clear it from the system's set of pending signals, and
return that signal number in the location referenced by
If prior to the call to
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
and the remainder shall remain queued. If no signal in
is pending at the time of the call, the thread shall be suspended
until one or more becomes pending. The signals defined by
shall have been blocked at the time of the call to
otherwise, the behavior is undefined. The effect of
on the signal actions for the signals in
If more than one thread is using
to wait for the same signal, no more than one of these threads shall
with the signal number. If more than a single thread is blocked in
for a signal when that signal is generated for the process, it is
unspecified which of the waiting threads returns from
If the signal is generated for a specific thread, as by
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.
Upon successful completion,
shall store the signal number of the received signal at the location
and return zero. Otherwise, an error number shall be returned to
indicate the error.
function may fail if:
argument contains an invalid or unsupported signal number.
The following sections are informative.
To provide a convenient way for a thread to wait for a signal, this volume of POSIX.1-2008
function. For most cases where a thread has to wait for a signal, the
function should be quite convenient, efficient, and adequate.
However, requests were made for a lower-level primitive than
and for semaphores that could be used by threads. After some
consideration, threads were allowed to use semaphores and
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,
should be used to handle the signal. Alternatively, if the
implementation provides semaphores, they also can be used, either
or from within a signal handling routine previously registered with
, Signal Concepts,
, Realtime Signals,
The Base Definitions volume of POSIX.1-2008,
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
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