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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
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
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
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
This man page illustrates the most common use of the futex(2)
primitives: it is by no means the only one.
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux
(proceedings of the Ottawa Linux Symposium 2002), futex example
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