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NAME
sigwait - 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, 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.
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 number.
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 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().
FUTURE DIRECTIONS
None.
SEE ALSO
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>
COPYRIGHT
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 .