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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.
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,  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)