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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. Which thread returns from sigwait() if more than a
single thread is waiting is unspecified.
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:
EINVAL 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 IEEE Std 1003.1-2001 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 and async-cancel-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
Signal Concepts , Realtime Signals , pause() , pthread_sigmask() ,
sigaction() , sigpending() , sigsuspend() , sigwaitinfo() , the Base
Definitions volume of IEEE Std 1003.1-2001, <signal.h>, <time.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .