SIGWAIT(3)                 POSIX Programmer's Manual                SIGWAIT(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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. 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.

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 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().

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.4, Signal Concepts, Section 2.8.1, Realtime Signals, pause(),
       pthread__sigmask(), sigaction(), sigpending(), sigsuspend(),
       sigtimedwait()

       The Base Definitions volume of POSIX.1-2017, <signal.h>, <time.h>

COPYRIGHT
       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           SIGWAIT(3)
