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, 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(3POSIX)