futex.2: Clarify that the FUTEX_WAIT timeout is relative

[thirdparty/man-pages.git] / man2 / futex.2

.\" 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 .
.\" 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 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
The
.I op
argument consists of two parts:
a command that specifies the operation to be performed,
bit-wise ORed with zero or or more options that
modify the behaviour of the operation.
The options that may be included in
.I op
are as follows:
.TP
.BR FUTEX_PRIVATE_FLAG " (since Linux 2.6.22)"
.\" commit 34f01cc1f512fa783302982776895c73714ebbc2
This option bit can be employed with all futex operations.
It tells the kernel that the futex is process private and not shared
with another process.
This allows the kernel to choose the fast path for validating
the user-space address and avoids expensive VMA lookups,
taking reference counts on file backing store, and so on.
.TP
.BR FUTEX_CLOCK_REALTIME " (since Linux 2.6.28)"
.\" commit 1acdac104668a0834cfa267de9946fac7764d486
This option bit can be employed only with the
.BR FUTEX_WAIT_BITSET
and
.BR FUTEX_WAIT_REQUEUE_PI
operations (described below).

If this option is set,
the kernel treats the user space supplied timeout as an absolute
time based on
.BR CLOCK_REALTIME .

If this option is not set,
the kernel treats the user space supplied timeout as relative time.
.\" FIXME a relative time based on what clock?
.PP
The operation specified in
.I op
is one of the following:
.TP
.BR FUTEX_WAIT " (since Linux 2.6.0)"
.\" Strictly speaking, since some time in 2.5.x
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 a relative timeout for 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
.BR FUTEX_WAIT_BITSET " (since Linux 2.6.25)"
.\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d
.\" FIXME TO complete
[As yet undocumented]
.TP
.BR FUTEX_WAKE " (since Linux 2.6.0)"
.\" Strictly speaking, since Linux 2.5.x
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_WAKE_OP " (since Linux 2.6.14)"
.\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721
.\" FIXME to complete
[As yet undocumented]
.TP
.BR FUTEX_WAKE_BITSET " (since Linux 2.6.25)"
.\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d
.\" FIXME TO complete
[As yet undocumented]
.TP
.BR FUTEX_LOCK_PI " (since Linux 2.6.18)"
.\" commit c87e2837be82df479a6bae9f155c43516d2feebc
.\" FIXME to complete
[As yet undocumented]
.TP
.BR FUTEX_UNLOCK_PI " (since Linux 2.6.18)"
.\" commit c87e2837be82df479a6bae9f155c43516d2feebc
.\" FIXME to complete
[As yet undocumented]
.TP
.BR FUTEX_TRYLOCK_PI " (since Linux 2.6.18)"
.\" commit c87e2837be82df479a6bae9f155c43516d2feebc
.\" FIXME to complete
[As yet undocumented]
.TP
.BR FUTEX_FD " (from Linux 2.6.0 up to and including Linux 2.6.25)"
.\" Strictly speaking, from Linux 2.5.x to 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 2.6.0)"
.\" Strictly speaking: from 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.
The argument
.I val
contains the number of waiters on
.I uaddr
that are immediately woken up.
The
.I timeout
argument is (ab)used to specify the number of waiters
that are requeued to the futex at
.IR uaddr2 ;
the kernel casts the
.I timeout
value to
.IR u32 .
.\" FIXME What are the constraints (if any) on the values of 'val' vs
.\" 'timeout' vs [the number of waites on 'uaddr']?

The argument
.I val3
is 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.
.\" FIXME should there be a statement in the description of FUTEX_REQUEUE
.\" to say that it should be avoided in favor of FUTEX_CMP_REQUEUE?
This operation 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 arguments
.IR val ,
.IR uaddr ,
.IR uaddr2 ,
and
.I timeout
are as for
.BR FUTEX_REQUEUE .
.TP
.BR FUTEX_CMP_REQUEUE_PI " (since Linux 2.6.31)"
.\" commit 52400ba946759af28442dee6265c5c0180ac7122
.\" FIXME to complete
[As yet undocumented]
.TP
.BR FUTEX_WAIT_REQUEUE_PI " (since Linux 2.6.31)"
.\" commit 52400ba946759af28442dee6265c5c0180ac7122
.\" FIXME to complete
[As yet undocumented]
.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 .
.\" FIXME: Is the following sentence correct?
(This probably indicates a race;
use the safe
.B FUTEX_WAKE
now.)
.TP
.B EFAULT
A required pointer argument (i.e.,
.IR uaddr ,
.IR uaddr2 ,
or
.IR timeout )
did not point to a valid user-space address.
.TP
.B EINTR
A
.B FUTEX_WAIT
operation was interrupted by a signal (see
.BR signal (7))
or a spurious wakeup.
.TP
.B EINVAL
.RB ( FUTEX_WAIT ,
.BR FUTEX_WAIT_REQUEUE_PI )
The supplied
.I timeout
argument was invalid
.RI ( tv_sec
was less than zero, or
.IR tv_nsec
was not less than 1000,000,000).
.TP
.B EINVAL
.RB ( FUTEX_WAIT ,
.BR FUTEX_WAKE ,
.BR FUTEX_REQUEUE ,
.BR FUTEX_CMP_REQUEUE )
.I uaddr
or (for
.BR FUTEX_REQUEUE 
and
.BR FUTEX_CMP_REQUEUE )
.I uaddr2
does not point to a valid object\(emthat is,
the address is not 4-byte-aligned.
.TP
.B EINVAL
.RB ( FUTEX_WAKE ,
.BR FUTEX_REQUEUE ,
.BR FUTEX_CMP_REQUEUE )
The kernel detected an inconsistency between the user-space state at
.I uaddr
and the kernel state\(emthat is, it detected a waiter which waits in
.BR FUTEX_LOCK_PI .
.TP
.B EINVAL
.RB ( FUTEX_REQUEUE )
.\" FIXME tglx suggested adding this, but does this error really
.\"       occur for FUTEX_REQUEUE?
.I uaddr
equals
.IR uaddr2
(i.e., an attempt was made to requeue to the same futex).
.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 ENOSYS
The
.BR FUTEX_CLOCK_REALTIME
option was specified in
.I op ,
but the accompanying operation was neither
.BR FUTEX_WAIT_BITSET
nor
.BR FUTEX_WAIT_REQUEUE_PI .
.TP
.B ETIMEDOUT
.RB ( FUTEX_WAIT )
The operation timed out.
.TP
.B EWOULDBLOCK
.RB ( FUTEX_WAIT )
The atomic enqueueing failed.
.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
Futexes were first made available in a stable kernel release
with Linux 2.6.0.

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