Document futex(2) with a lot of inputs from schwarze@

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+.\" $OpenBSD: futex.2,v 1.1 2017/04/28 14:02:57 mpi Exp $
+.\"
+.\" Copyright (c) 2017 Martin Pieuchot
+.\"
+.\" Permission to use, copy, modify, and distribute this software for any
+.\" purpose with or without fee is hereby granted, provided that the above
+.\" copyright notice and this permission notice appear in all copies.
+.\"
+.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+.\"
+.Dd $Mdocdate: April 28 2017 $
+.Dt FUTEX 2
+.Os
+.Sh NAME
+.Nm futex
+.Nd fast userspace locking primitive
+.Sh SYNOPSIS
+.In sys/time.h
+.In sys/futex.h
+.Ft int
+.Fo futex
+.Fa "volatile uint32_t *uaddr"
+.Fa "int op"
+.Fa "int val"
+.Fa "const struct timespec *timeout"
+.Fa "volatile uint32_t *uaddr2"
+.Fc
+.Sh DESCRIPTION
+The
+.Fn futex
+syscall provides sleep and wakeup primitives related to a particular address.
+.Pp
+Three
+.Fa op
+operations are currently supported:
+.Bl -tag -width FUTEX_REQUEUE -offset indent
+.It Dv FUTEX_WAIT
+If
+.Fa val
+is equal to
+.Pf * Fa uaddr ,
+the calling thread is blocked on the
+.Dq wait channel
+identified by
+.Fa uaddr
+until
+.Fa timeout
+expires or until another thread issues a
+.Dv FUTEX_WAKE
+or
+.Dv FUTEX_REQUEUE
+operation with the same
+.Fa uaddr
+address.
+.Fa uaddr2
+is ignored.
+.It Dv FUTEX_WAKE
+Unblocks
+.Fa val
+threads sleeping on the
+wait channel identified by
+.Fa uaddr .
+.Fa timeout
+and
+.Fa uaddr2
+are ignored.
+.It Dv FUTEX_REQUEUE
+Similar to
+.Dv FUTEX_WAKE
+but also requeue remaining threads from the wait channel
+.Fa uaddr
+to
+.Fa uaddr2 .
+In this case, pass
+.Fa "uint32_t val2"
+as the fourth argument instead of
+.Fa timeout .
+ At most that number of threads is requeued.
+.El
+.Sh RETURN VALUES
+For
+.Dv FUTEX_WAKE
+and
+.Dv FUTEX_REQUEUE ,
+.Fn futex
+returns the number of woken threads.
+.Pp
+For
+.Dv FUTEX_WAIT ,
+.Fn futex
+returns zero if woken by a matching
+.Dv FUTEX_WAKE
+or
+.Dv FUTEX_REQUEUE
+call, otherwise an error number is returned to indicate the error.
+.Sh ERRORS
+.Fn futex
+will fail if:
+.Bl -tag -width Er
+.It Bq Er ENOSYS
+The
+.Fa op
+argument is invalid.
+.It Bq Er EFAULT
+The userspace address
+.Fa uaddr
+is invalid.
+.It Bq Er EAGAIN
+The value pointed to by
+.Fa uaddr
+is not the same as the expected value
+.Fa val .
+.It Bq Er ETIMEDOUT
+The
+.Fa timeout
+expired before the thread was woken up
+.It Bq Er EINTR
+A signal arrived.
+.El
+.Sh SEE ALSO
+.Xr pthread_cond_wait 3 ,
+.Xr pthread_mutex_lock 3 ,
+.Xr tsleep 9
+.Sh HISTORY
+The
+.Fn futex
+syscall first appeared in Linux 2.5.7 and was added to
+.Ox 6.2 .