FUTEX(2) FUTEX(2)
NAME
futex - Fast Userspace Locking system call
SYNOPSIS
#include<linux/futex.h>
#include<sys/time.h>
intsys_futex(void*futex,int op,int val,conststructtimespec
*timeout);
DESCRIPTION
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 mem-
ory in separate processes may not be equal, the kernel maps them inter-
nally so the same memory mapped in different locations will correspond
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 arbitrate. 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 assem-
bly 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:
FUTEX_WAIT
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 con-
tents describe the maximum duration of the wait, which is infi-
nite otherwise. For futex(4), this call is executed if decre-
menting the count gave a negative value (indicating contention),
and will sleep until another process releases the futex and
executes the FUTEX_WAKE operation.
FUTEX_WAKE
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 avail-
able).
FUTEX_FD
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.
RETURNVALUE
Depending on which operation was executed, the returned value can have
differing meanings.
FUTEX_WAIT
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.
FUTEX_WAKE
Returns the number of processes woken up.
FUTEX_FD
Returns the new file descriptor associated with the futex.
NOTES
To reiterate, bare futexes are not intended as an easy to use abstrac-
tion for end-users. Implementors are expected to be assembly literate
and to have read the sources of the futex userspace library referenced
below.
AUTHORS
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.
VERSIONS
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.
SEEALSO
futex(4), 'Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux'
(proceedings of the Ottawa Linux Symposium 2002), futex example
library, futex-*.tar.bz2 <URL:ftp://ftp.nl.kernel.org:/pub/linux/ker-
nel/people/rusty/>.
31 December 2002 FUTEX(2)
Take our 
