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Subjectsyscall documentation (2)
The next new page is futex.2.

Comments welcome.

futex - Fast Userspace Locking system call

#include <linux/futex.h>

#include <sys/time.h>

int sys_futex (void *futex, int op, int val, const struct
timespec *timeout);

The sys_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 pro­
cesses may not be equal, the kernel maps them internally
so the same memory mapped in different locations will cor­
respond for sys_futex calls). It is typically used to
implement the contended case of a lock in shared memory,
as described in futex(4).

When a futex(4) operation did not finish uncontended in
userspace, a call needs to be made to the kernel to arbi­
trate. Arbitration can either mean putting the calling
process to sleep or, conversely, waking a waiting process.

Callers of this function are expected to adhere to the
semantics as set out in futex(4). As these semantics
involve writing non-portable assembly instructions, this
in turn probably means that most users will in fact be
library authors and not general application developers.

The futex argument needs to point to an aligned integer
which stores the counter. The operation to execute is
passed via the op parameter, along with a value val.

Three operations are currently defined:

This operation atomically verifies that the futex
address still contains the value given, and sleeps
awaiting FUTEX_WAKE on this futex address. If the
timeout argument is non-NULL, its contents describe
the maximum duration of the wait, which is infinite
otherwise. For futex(4), this call is executed if
decrementing the count gave a negative value (indi­
cating contention), and will sleep until another
process releases the futex and executes the
FUTEX_WAKE operation.

This operation wakes at most val processes waiting
on this futex address (ie. inside FUTEX_WAIT). For
futex(4), 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).

To support a asynchronous wakeups, this operation
associates a file descriptor with a futex. If
another process executes a FUTEX_WAKE, the process
will receive the signal number that was passed in
val. The calling process must close the returned
file descriptor after use.

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

Depending on which operation was executed, the returned
value can have differing meanings.

Returns 0 if the process was woken by a FUTEX_WAKE
call. In case of timeout, ETIMEDOUT is returned. If
the futex was not equal to the expected value, the
operation returns EWOULDBLOCK. Signals (or other
spurious wakeups) cause FUTEX_WAIT to return EINTR.

Returns the number of processes woken up.

Returns the new file descriptor associated with the

To reiterate, bare futexes are not intended as an easy to
use abstraction for end-users. Implementors are expected
to be assembly literate and to have read the sources of
the futex userspace library referenced below.

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 Tech­
nology Center). This page written by bert hubert.

Initial futex support was merged in Linux 2.5.7 but with
different semantics from those described above. Current
semantics are available from Linux 2.5.40 onwards.

futex(4), `Fuss, Futexes and Furwocks: Fast Userlevel
Locking in Linux' (proceedings of the Ottawa Linux Sympo­
sium 2002), futex example library, futex-*.tar.bz2

31 December 2002 FUTEX(2)
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