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.
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 sigaction().
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