SIGWAIT(3P) POSIX Programmer's Manual SIGWAIT(3P)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.
NAMEsigwait — 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, atomi‐
cally 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 sig‐
nal 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 sig‐
nals defined by set shall have been blocked at the time of the call to
sigwait(); otherwise, the behavior is undefined. The effect of sig‐
wait() 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 unspeci‐
fied which of the waiting threads returns from sigwait(). If the sig‐
nal 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 selec‐
tion order between realtime and non-realtime signals, or between multi‐
ple 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 num‐
ber.
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.1‐2008 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 considera‐
tion, 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 asyn‐
chronous 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 sig‐
nal 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(), sigtimed‐
wait()
The Base Definitions volume of POSIX.1‐2008, <signal.h>, <time.h>
COPYRIGHT
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 Electri‐
cal 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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SIGWAIT(3P)
