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FUTEX(2) FUTEX(2)
NAME
futex - Fast Userspace Locking system call
SYNOPSIS
#include <linux/futex.h>
#include <sys/time.h>
int sys_futex (void *futex, int op, int val, const struct timespec *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 memory in separate processes may not be
equal, the kernel maps them internally so the same memory mapped in different loca-
tions 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 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:
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 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 (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 available).
FUTEX_FD
To support 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.
RETURN VALUE
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.
ERRORS
EFAULT Error in getting timeout information from userspace.
EINVAL An operation was not defined or error in page alignment.
NOTES
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.
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.
SEE ALSO
futex(4), ‘Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux’ (proceed-
ings of the Ottawa Linux Symposium 2002), futex example library, futex-*.tar.bz2
<URL:ftp://ftp.nl.kernel.org:/pub/linux/kernel/people/rusty/>.
31 December 2002 FUTEX(2)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
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