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 (con-
       sult 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, atomically clear it from the system's set of pending sig-
       nals, and return that signal number in the location referenced by sig. If prior to the call to sigwait() there are multi-
       ple  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 multi-
       ple  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 sig-
       wait(); 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,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                   SIGWAIT(3P)