noble (7) futex.7.gz
NAME
futex - fast user-space locking
SYNOPSIS
#include <linux/futex.h>
DESCRIPTION
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 mutexes, condition variables, read-write locks, barriers, and semaphores.
Most programmers will in fact not be using futexes directly but will instead rely on system libraries
built on them, such as the Native POSIX Thread Library (NPTL) (see pthreads(7)).
A futex is identified by a piece of memory which can be shared between processes or threads. In these
different processes, the futex 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 occurs entirely in user space for the noncontended case. The kernel is involved only 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 touched only by atomic assembler instructions.
This integer is four bytes long on all platforms. 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.
Semantics
Any futex operation starts in user space, but it may be 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. User space 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 semantics are available from Linux 2.5.40 onward.
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 user-space library
referenced below.
This man page illustrates the most common use of the futex(2) primitives; it is by no means the only one.
SEE ALSO
clone(2), futex(2), get_robust_list(2), set_robust_list(2), set_tid_address(2), pthreads(7) Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceedings of the Ottawa Linux Symposium 2002), futex example library, futex-*.tar.bz2 ⟨https://mirrors.kernel.org/pub/linux/kernel/people /rusty/⟩.