FUTEX(7) Linux Programmer's Manual FUTEX(7)
futex - Fast Userspace Locking
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 devel-
opment. Most programmers will in fact not be using futexes directly
but instead rely on system libraries built on them, such as the NPTL
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 seman-
tics; it is a counter that can be incremented and decremented atomi-
cally; processes can wait for the value to become positive.
Futex operation is entirely userspace for the non-contended case. The
kernel is only involved to arbitrate the contended case. As any sane
design will strive for non-contention, futexes are also optimized for
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 commu-
nicate 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. Afterwards,
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 non-contended 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 wait-
ers. 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 circum-
stances, 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 wait-
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.
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
This man page illustrates the most common use of the futex(2) primi-
tives: it is by no means the only one.
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceed-
ings of the Ottawa Linux Symposium 2002), futex example library,
This page is part of release 3.05 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)