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FUTEX(7) Linux Programmer's Manual FUTEX(7)

NAME
futex - Fast Userspace Locking

SYNOPSIS
#include <linux/futex.h>

DESCRIPTION
The Linux kernel provides futexes ("Fast Userspace muTexes") as a building block for fast userspace locking and sema-
phores. 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 pro-
cesses, 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 appli-
cation is commonly called multithreaded.

Semantics
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 inte-
ger. 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 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.

VERSIONS
Initial futex support was merged in Linux 2.5.7 but with different semantics from those described above. Current seman-
tics are available from Linux 2.5.40 onwards.

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.

This man page illustrates the most common use of the futex(2) primitives: it is by no means the only one.