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FUTEX(7) Linux Programmer's Manual FUTEX(7)
futex - Fast Userspace Locking
#include <linux/futex.h>
The Linux kernel provides futexes ("Fast Userspace muTexes") as a building
block for fast userspace locking and semaphores. Futexes are very basic and
lend themselves well for building higher level locking abstractions such as
POSIX mutexes.
This page does not set out to document all design decisions but restricts
itself to issues relevant for application and library development. Most
programmers will in fact not be using futexes directly but instead rely on
system libraries built on them, such as the NPTL pthreads implementation.
A futex is identified by a piece of memory which can be shared between
different processes. In these different processes, it need not have identical
addresses. In its bare form, a futex has semaphore semantics; it is a counter
that can be incremented and decremented atomically; processes can wait for the
value to become positive.
Futex operation is entirely userspace for the noncontended case. The kernel
is only involved to arbitrate the contended case. As any sane design will
strive for noncontention, futexes are also optimized for this situation.
In its bare form, a futex is an aligned integer which is only touched by
atomic assembler instructions. Processes can share this integer using
mmap(2), via shared memory segments or because they share memory space, in
which case the application is commonly called multithreaded.
Any futex operation starts in userspace, but it may necessary to communicate
with the kernel using the futex(2) system call.
To "up" a futex, execute the proper assembler instructions that will cause the
host CPU to atomically increment the integer. Afterward, check if it has in
fact changed from 0 to 1, in which case there were no waiters and the
operation is done. This is the noncontended case which is fast and should be
common.
In the contended case, the atomic increment changed the counter from -1 (or
some other negative number). If this is detected, there are waiters.
Userspace should now set the counter to 1 and instruct the kernel to wake up
any waiters using the FUTEX_WAKE operation.
Waiting on a futex, to "down" it, is the reverse operation. Atomically
decrement the counter and check if it changed to 0, in which case the
operation is done and the futex was uncontended. In all other circumstances,
the process should set the counter to -1 and request that the kernel wait for
another process to up the futex. This is done using the FUTEX_WAIT operation.
The futex(2) system call can optionally be passed a timeout specifying how
long the kernel should wait for the futex to be upped. In this case,
semantics are more complex and the programmer is referred to futex(2) for more
details. The same holds for asynchronous futex waiting.
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
onward.
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.
This man page illustrates the most common use of the futex(2) primitives: it
is by no means the only one.
futex(2)
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.kernel.org/pub/linux/kernel/people/rusty/>.
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2002-12-31 FUTEX(7)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface