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.Dd $Mdocdate: May 26 2021 $
.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, a value of \-1 is returned and
.Va errno
is set 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 EINVAL
The
.Fa timeout
specified a second value less than zero,
or a nanosecond value less than zero or greater than or equal to 1000 million.
.It Bq Er ETIMEDOUT
The
.Fa timeout
expired before the thread was woken up
.It Bq Er EINTR
A signal arrived.
.It Bq Er ECANCELED
A signal arrived and
.Fa SA_RESTART
was set.
.El
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr pthread_cond_wait 3 ,
.Xr pthread_mutex_lock 3 ,
.Xr tsleep 9
.Rs
.%A Ulrich Drepper
.%T Futexes Are Tricky
.%U https://www.akkadia.org/drepper/futex.pdf
.%D November 5, 2011
.Re
.Sh HISTORY
The
.Fn futex
syscall first appeared in Linux 2.5.7 and was added to
.Ox 6.2 .