man-pages/man2/futex.2

.\" Page by b.hubert - may be freely modified and distributed
.\"
.\" 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
.\" 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.
.\" 2.6.25 adds FUTEX_WAKE_BITSET, FUTEX_WAIT_BITSET
.\"
.TH FUTEX 2 2012-08-05 "Linux" "Linux Programmer's Manual"
.SH NAME
futex \- Fast Userspace Locking system call
.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
.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 userspace, 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 describe the maximum
duration of the wait, which is infinite otherwise.
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 .
The argument
.I timeout
is ignored.
.SH "RETURN VALUE"
.PP
Depending on which operation was executed,
the returned value for a successful call can have differing meanings.
.TP
.B FUTEX_WAIT
Returns 0 if the process was woken by a
.B FUTEX_WAKE
call.
.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.
.PP
In the event of an error, all operations return \-1, and set
.I errno
to indicate the error.
.SH ERRORS
.TP
.B EACCES
No read access to futex memory.
.TP
.B EAGAIN
.B FUTEX_CMP_REQUEUE
detect that the value at
.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
.I timeout
information from userspace.
.TP
.B EINTR
.B FUTEX_WAIT
operation was interrupted by signal (see
.BR signal (7)
) or by other spurious wakeup.
.TP
.B EINVAL
An operation was not defined or error in page alignment.
.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
The value, pointed to by
.I uaddr
was not equal to the expected value
.I val
at the moment of calling
.B FUTEX_WAIT
operation.
.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 userspace 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 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.nl.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/
.UE