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FreeBSD 11.0 - man page for sigwait (freebsd section 2)

SIGWAIT(2)						      BSD System Calls Manual							SIGWAIT(2)

NAME
sigwait -- select a set of signals
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <signal.h> int sigwait(const sigset_t * restrict set, int * restrict sig);
DESCRIPTION
The sigwait() system call selects a set of signals, specified by set. If none of the selected signals are pending, sigwait() waits until one or more of the selected signals has been generated. Then sigwait() atomically clears one of the selected signals from the set of pending signals (for the process or for the current thread) and sets the location pointed to by sig to the signal number that was cleared. The signals specified by set should be blocked at the time of the call to sigwait(). If more than one thread is using sigwait() to wait for the same signal, no more than one of these threads will 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 will return. Should any of the multiple pending signals in the range SIGRTMIN to SIGRTMAX be selected, it will be the lowest numbered one. The selection order between realtime and non-realtime signals, or between multiple pending non-realtime signals, is unspecified.
IMPLEMENTATION NOTES
The sigwait() function is implemented as a wrapper around the __sys_sigwait() system call, which retries the call on EINTR error.
RETURN VALUES
If successful, sigwait() returns 0 and sets the location pointed to by sig to the cleared signal number. Otherwise, an error number is returned.
ERRORS
The sigwait() system call will fail if: [EINVAL] The set argument specifies one or more invalid signal numbers.
SEE ALSO
sigaction(2), sigpending(2), sigqueue(2), sigsuspend(2), sigtimedwait(2), sigwaitinfo(2), pause(3), pthread_sigmask(3)
STANDARDS
The sigwait() function conforms to ISO/IEC 9945-1:1996 (``POSIX.1'').
BSD
September 6, 2013 BSD