path: root/man2/futex.2

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.\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE)
.\" may be freely modified and distributed
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.\" Niki A. Rahimi (LTC Security Development, narahimi@us.ibm.com)
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.\" Modified 2004-06-17 mtk
.\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE
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.\" FIXME
.\" See also https://bugzilla.kernel.org/show_bug.cgi?id=14303
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.\"	Author: Jakub Jelinek <jakub@redhat.com>
.\"	Date:   Tue Sep 6 15:16:25 2005 -0700
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.\" 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>
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.\"	Author: Ingo Molnar <mingo@elte.hu>
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.\" 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
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.\" 2.6.31 adds FUTEX_WAIT_REQUEUE_PI, FUTEX_CMP_REQUEUE_PI
.\"	commit 52400ba946759af28442dee6265c5c0180ac7122
.\"	Author: Darren Hart <dvhltc@us.ibm.com>
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.\"	Author: Darren Hart <dvhltc@us.ibm.com>
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.\"	See Documentation/futex-requeue-pi.txt
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.TH FUTEX 2 2014-05-21 "Linux" "Linux Programmer's Manual"
.SH NAME
futex \- fast user-space locking
.SH SYNOPSIS
.nf
.sp
.B "#include <linux/futex.h>"
.B "#include <sys/time.h>"
.sp
.BI "int futex(int *" uaddr ", int " op ", int " val \
", const struct timespec *" timeout ,
.br
.BI "          int *" uaddr2 ", int " val3 );
.\" int *? void *? u32 *?
.fi
.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
.PP
The
.BR futex ()
system call provides a method for
a program to wait for a value at a given address to change, and a
method to wake up anyone waiting on a particular address (while the
addresses for the same memory in separate processes may not be
equal, the kernel maps them internally so the same memory mapped in
different locations will correspond for
.BR futex ()
calls).
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 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.
.PP
Callers of this function are expected to adhere to the semantics as set out in
.BR futex (7).
As these
semantics involve writing nonportable assembly instructions, this in turn
probably means that most users will in fact be library authors and not
general application developers.
.PP
The
.I uaddr
argument needs to point to an aligned integer which stores the counter.
The operation to execute is passed via the
.I op
argument, along with a value
.IR val .
.PP
Five operations are currently defined:
.TP
.B FUTEX_WAIT
This operation atomically verifies that the futex address
.I uaddr
still contains the value
.IR val ,
and sleeps awaiting
.B FUTEX_WAKE
on this futex address.
If the
.I timeout
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
.I val3
are ignored.

For
.BR futex (7),
this call is executed if decrementing the count gave a negative value
(indicating contention), and will sleep until another process releases
the futex and executes the
.B FUTEX_WAKE
operation.
.TP
.B FUTEX_WAKE
This operation wakes at most \fIval\fP
processes waiting on this futex address (i.e., inside
.BR FUTEX_WAIT ).
The arguments
.IR timeout ,
.I uaddr2
and
.I val3
are ignored.

For
.BR futex (7),
this is executed if incrementing
the count showed that there were waiters, once the futex value has been set
to 1 (indicating that it is available).
.TP
.BR FUTEX_FD " (present up to and including Linux 2.6.25)"
To support asynchronous wakeups, this operation associates a file descriptor
with a futex.
.\" , suitable for .BR poll (2).
If another process executes a
.BR FUTEX_WAKE ,
the process will receive the signal number that was passed in
.IR val .
The calling process must close the returned file descriptor after use.
The arguments
.IR timeout ,
.I uaddr2
and
.I val3
are ignored.

To prevent race conditions, the caller should test if the futex has
been upped after
.B FUTEX_FD
returns.

Because it was inherently racy,
.B FUTEX_FD
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
when
.B FUTEX_WAKE
is used and all processes woken up need to acquire another futex.
This call wakes up
.I val
processes, and requeues all other waiters on the futex at address
.IR uaddr2 .
The arguments
.I timeout
and
.I val3
are ignored.
.TP
.BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)"
There was a race in the intended use of
.BR FUTEX_REQUEUE ,
so
.B FUTEX_CMP_REQUEUE
was introduced.
This is similar to
.BR FUTEX_REQUEUE ,
but first checks whether the location
.I uaddr
still contains the value
.IR val3 .
If not, the operation fails with the error
.BR EAGAIN .
The argument
.I timeout
is ignored.
.SH RETURN VALUE
.PP
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.
See ERRORS for the various possible error returns.
.TP
.B FUTEX_WAKE
Returns the number of processes woken up.
.TP
.B FUTEX_FD
Returns the new file descriptor associated with the futex.
.TP
.B FUTEX_REQUEUE
Returns the number of processes woken up.
.TP
.B FUTEX_CMP_REQUEUE
Returns the number of processes woken up.
.SH ERRORS
.TP
.B EACCES
No read access to futex memory.
.TP
.B EAGAIN
.B FUTEX_CMP_REQUEUE
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 retrieving
.I timeout
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
Invalid argument.
.TP
.B ENFILE
The system limit on the total number of open files has been reached.
.TP
.B ENOSYS
Invalid operation specified in
.IR op .
.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
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\(emmessy, especially
on the s390 architecture.
.SH CONFORMING TO
This system call is Linux-specific.
.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 user-space library referenced below.
.\" .SH "AUTHORS"
.\" .PP
.\" Futexes were designed and worked on by
.\" Hubertus Franke (IBM Thomas J. Watson Research Center),
.\" Matthew Kirkwood, Ingo Molnar (Red Hat)
.\" and Rusty Russell (IBM Linux Technology Center).
.\" This page written by bert hubert.
.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
.UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf
.UE
.PP
Futex example library, futex-*.tar.bz2 at
.br
.UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/
.UE