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.
sigwait — wait for queued signals
int sigwait(const sigset_t *restrict set, int *restrict sig);
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.
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.
The sigwait() function may fail if:
- The set argument contains an invalid or unsupported signal
The following sections are informative.
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().
Section 2.4, Signal Concepts, Section 2.8.1, Realtime
Signals, pause(), pthread_sigmask(),
The Base Definitions volume of POSIX.1‐2017,
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