-.\" Page by b.hubert - may be freely modified and distributed
+.\" Page by b.hubert
+.\"
+.\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE)
+.\" may be freely modified and distributed
+.\" %%%LICENSE_END
.\"
.\" Niki A. Rahimi (LTC Security Development, narahimi@us.ibm.com)
.\" added ERRORS section.
.\" Modified 2004-06-17 mtk
.\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE
.\"
-.\" FIXME
+.\" FIXME .
+.\" See also https://bugzilla.kernel.org/show_bug.cgi?id=14303
.\" 2.6.14 adds FUTEX_WAKE_OP
+.\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721
+.\" Author: Jakub Jelinek <jakub@redhat.com>
+.\" Date: Tue Sep 6 15:16:25 2005 -0700
+.\"
+.\" FIXME .
.\" 2.6.18 adds (Ingo Molnar) priority inheritance support:
.\" FUTEX_LOCK_PI, FUTEX_UNLOCK_PI, and FUTEX_TRYLOCK_PI. These need
.\" to be documented in the manual page. Probably there is sufficient
.\" material in the kernel source file Documentation/pi-futex.txt.
+.\" commit c87e2837be82df479a6bae9f155c43516d2feebc
+.\" Author: Ingo Molnar <mingo@elte.hu>
+.\" Date: Tue Jun 27 02:54:58 2006 -0700
+.\"
+.\" commit e2970f2fb6950183a34e8545faa093eb49d186e1
+.\" Author: Ingo Molnar <mingo@elte.hu>
+.\" Date: Tue Jun 27 02:54:47 2006 -0700
+.\"
+.\" See Documentation/pi-futex.txt
+.\"
+.\" FIXME .
.\" 2.6.25 adds FUTEX_WAKE_BITSET, FUTEX_WAIT_BITSET
+.\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d
+.\" Author: Thomas Gleixner <tglx@linutronix.de>
+.\" Date: Fri Feb 1 17:45:14 2008 +0100
+.\"
+.\" FIXME .
+.\" 2.6.31 adds FUTEX_WAIT_REQUEUE_PI, FUTEX_CMP_REQUEUE_PI
+.\" commit 52400ba946759af28442dee6265c5c0180ac7122
+.\" Author: Darren Hart <dvhltc@us.ibm.com>
+.\" Date: Fri Apr 3 13:40:49 2009 -0700
+.\"
+.\" commit ba9c22f2c01cf5c88beed5a6b9e07d42e10bd358
+.\" Author: Darren Hart <dvhltc@us.ibm.com>
+.\" Date: Mon Apr 20 22:22:22 2009 -0700
+.\"
+.\" See Documentation/futex-requeue-pi.txt
.\"
-.TH FUTEX 2 2010-05-22 "Linux" "Linux Programmer's Manual"
+.TH FUTEX 2 2014-05-21 "Linux" "Linux Programmer's Manual"
.SH NAME
-futex \- Fast Userspace Locking system call
+futex \- fast user-space locking
.SH SYNOPSIS
.nf
.sp
.BI " int *" uaddr2 ", int " val3 );
.\" int *? void *? u32 *?
.fi
-.SH "DESCRIPTION"
+.IR Note :
+There is no glibc wrapper for this system call; see NOTES.
+.SH DESCRIPTION
.PP
The
.BR futex ()
different locations will correspond for
.BR futex ()
calls).
-It is typically used to
+This system call is typically used to
implement the contended case of a lock in shared memory, as
described in
.BR futex (7).
.PP
When a
.BR futex (7)
-operation did not finish uncontended in userspace, a call needs to be made
+operation did not finish uncontended in user space, a call needs to be made
to the kernel to arbitrate.
Arbitration can either mean putting the calling
process to sleep or, conversely, waking a waiting process.
on this futex address.
If the
.I timeout
-argument is non-NULL, its contents describe the maximum
-duration of the wait, which is infinite otherwise.
+argument is non-NULL, its contents specify the duration of the wait.
+(This interval will be rounded up to the system clock granularity,
+and kernel scheduling delays mean that the
+blocking interval may overrun by a small amount.)
+If
+.I timeout
+is NULL, the call blocks indefinitely.
The arguments
.I uaddr2
and
Because it was inherently racy,
.B FUTEX_FD
-has been removed from Linux 2.6.26 onwards.
+has been removed from Linux 2.6.26 onward.
.TP
.BR FUTEX_REQUEUE " (since Linux 2.5.70)"
This operation was introduced in order to avoid a "thundering herd" effect
The argument
.I timeout
is ignored.
-.SH "RETURN VALUE"
+.SH RETURN VALUE
.PP
-Depending on which operation was executed,
-the returned value for a successful call can have differing meanings.
+In the event of an error, all operations return \-1, and set
+.I errno
+to indicate the error.
+The return value on success depends on the operation,
+as described in the following list:
.TP
.B FUTEX_WAIT
Returns 0 if the process was woken by a
.B FUTEX_WAKE
call.
-In case of timeout,
-the operation fails with the error
-.BR ETIMEDOUT .
-If the futex was not equal to the expected value,
-the operation fails with the error
-.BR EWOULDBLOCK .
-Signals (see
-.BR signal (7))
-or other spurious wakeups cause
-.B FUTEX_WAIT
-to fail with the error
-.BR EINTR .
+See ERRORS for the various possible error returns.
.TP
.B FUTEX_WAKE
Returns the number of processes woken up.
.TP
.B FUTEX_CMP_REQUEUE
Returns the number of processes woken up.
-.PP
-In the event of an error, all operations return \-1, and set
-.I errno
-to indicate the error.
.SH ERRORS
.TP
.B EACCES
.TP
.B EAGAIN
.B FUTEX_CMP_REQUEUE
-found an unexpected futex value.
+detected that the value pointed to by
+.I uaddr
+is not equal to the expected value
+.IR val3 .
(This probably indicates a race;
use the safe
.B FUTEX_WAKE
now.)
.TP
.B EFAULT
-Error in getting
+Error retrieving
.I timeout
-information from userspace.
+information from user space.
+.TP
+.B EINTR
+A
+.B FUTEX_WAIT
+operation was interrupted by a signal (see
+.BR signal (7))
+or a spurious wakeup.
.TP
.B EINVAL
-An operation was not defined or error in page alignment.
+Invalid argument.
.TP
.B ENFILE
The system limit on the total number of open files has been reached.
.B ENOSYS
Invalid operation specified in
.IR op .
-.SH "VERSIONS"
+.TP
+.B ETIMEDOUT
+Timeout during the
+.B FUTEX_WAIT
+operation.
+.TP
+.B EWOULDBLOCK
+.I op
+was
+.BR FUTEX_WAIT
+and the value pointed to by
+.I uaddr
+was not equal to the expected value
+.I val
+at the time of the call.
+.SH VERSIONS
.PP
Initial futex support was merged in Linux 2.5.7 but with different semantics
from what was described above.
A 4-argument system call with the semantics
-given here was introduced in Linux 2.5.40.
-In Linux 2.5.70 one argument
+described in this page was introduced in Linux 2.5.40.
+In Linux 2.5.70, one argument
was added.
-In Linux 2.6.7 a sixth argument was added \(em messy, especially
+In Linux 2.6.7, a sixth argument was added\(emmessy, especially
on the s390 architecture.
-.SH "CONFORMING TO"
+.SH CONFORMING TO
This system call is Linux-specific.
-.SH "NOTES"
+.SH NOTES
.PP
To reiterate, bare futexes are not intended as an easy-to-use abstraction
for end-users.
(There is no wrapper function for this system call in glibc.)
Implementors are expected to be assembly literate and to have
-read the sources of the futex userspace library referenced below.
+read the sources of the futex user-space library referenced below.
.\" .SH "AUTHORS"
.\" .PP
.\" Futexes were designed and worked on by
.\" Matthew Kirkwood, Ingo Molnar (Red Hat)
.\" and Rusty Russell (IBM Linux Technology Center).
.\" This page written by bert hubert.
-.SH "SEE ALSO"
+.SH SEE ALSO
+.BR restart_syscall (2),
.BR futex (7)
.PP
\fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP
(proceedings of the Ottawa Linux Symposium 2002), online at
.br
-http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf
+.UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf
+.UE
.PP
Futex example library, futex-*.tar.bz2 at
.br
-ftp://ftp.nl.kernel.org/pub/linux/kernel/people/rusty/.
+.UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/
+.UE
+.SH COLOPHON
+This page is part of release 3.79 of the Linux
+.I man-pages
+project.
+A description of the project,
+information about reporting bugs,
+and the latest version of this page,
+can be found at
+\%http://www.kernel.org/doc/man\-pages/.
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