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.\" $FreeBSD: releng/11.0/lib/libc/sys/sigwait.2 255296 2013-09-06 09:08:40Z jilles $
.\"
.Dd September 6, 2013
.Dt SIGWAIT 2
.Os
.Sh NAME
.Nm sigwait
.Nd select a set of signals
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In signal.h
.Ft int
.Fn sigwait "const sigset_t * restrict set" "int * restrict sig"
.Sh DESCRIPTION
The
.Fn sigwait
system call selects a set of signals, specified by
.Fa set .
If none of the selected signals are pending,
.Fn sigwait
waits until one or more of the selected signals has been generated.
Then
.Fn 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
.Fa sig
to the signal number that was cleared.
.Pp
The signals specified by
.Fa set
should be blocked at the time of the call to
.Fn sigwait .
.Pp
If more than one thread is using
.Fn sigwait
to wait for the same signal, no more than one of these threads will return from
.Fn sigwait
with the signal number.
If more than a single thread is blocked in
.Fn sigwait
for a signal when that signal is generated for the process, it is unspecified
which of the waiting threads returns from
.Fn sigwait .
If the signal is generated for a specific thread, as by
.Fn pthread_kill ,
only that thread will return.
.Pp
Should any of the multiple pending signals in the range
.Dv SIGRTMIN
to
.Dv 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.
.Sh IMPLEMENTATION NOTES
The
.Fn sigwait
function is implemented as a wrapper around the
.Fn __sys_sigwait
system call, which retries the call on
.Er EINTR
error.
.Sh RETURN VALUES
If successful,
.Fn sigwait
returns 0 and sets the location pointed to by
.Fa sig
to the cleared signal number.
Otherwise, an error number is returned.
.Sh ERRORS
The
.Fn sigwait
system call will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa set
argument
specifies one or more invalid signal numbers.
.El
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr sigpending 2 ,
.Xr sigqueue 2 ,
.Xr sigsuspend 2 ,
.Xr sigtimedwait 2 ,
.Xr sigwaitinfo 2 ,
.Xr pause 3 ,
.Xr pthread_sigmask 3
.Sh STANDARDS
The
.Fn sigwait
function conforms to
.St -p1003.1-96 .