futex - fast user-space locking
The Linux kernel provides futexes ("Fast user-space mutexes") as a
building block for fast user-space 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
Futex operation is entirely user space 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 user space, but it may necessary to communicate
with the kernel using the futex(2)
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
In the contended case, the atomic increment changed the counter from -1 (or some
other negative number). If this is detected, there are waiters. User space
should now set the counter to 1 and instruct the kernel to wake up any waiters
using the FUTEX_WAKE
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
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
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 user-space library referenced below.
This man page illustrates the most common use of the futex(2)
it is by no means the only one.
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux
of the Ottawa Linux Symposium 2002), futex example library, futex-*.tar.bz2
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found