In computing, a futex (short for "fast userspace mutex") is a kernel system call that programmers can use to implement basic locking, or as a building block for higher-level locking abstractions such as semaphores and POSIX mutexes or condition variables.
A futex consists of a kernelspace wait queue that is attached to an atomic integer in userspace. Multiple processes or threads operate on the integer entirely in userspace (using atomic operations to avoid interfering with one another), and only resort to relatively expensive system calls to request operations on the wait queue (for example to wake up waiting processes, or to put the current process on the wait queue). A properly programmed futex-based lock will not use system calls except when the lock is contended; since most operations do not require arbitration between processes, this will not happen in most cases.
On Linux, Hubertus Franke (IBM Thomas J. Watson Research Center), Matthew Kirkwood, Ingo Molnár (Red Hat) and Rusty Russell (IBM Linux Technology Center) originated the futex mechanism. Futexes appeared for the first time in version 2.5.7 of the Linux kernel development series; the semantics stabilized as of version 2.5.40, and futexes have been part of the Linux kernel mainline since the December 2003 release of 2.6.x stable kernel series.
In 2002 discussions took place on a proposal to make futexes accessible via the file system by creating a special node in
/proc. However, Linus Torvalds strongly opposed this idea and rejected any related patches.
Futexes have been implemented in Microsoft Windows since Windows 8 or Windows Server 2012 under the name WaitOnAddress.
In 2013 Microsoft patented futexes and the patent was granted in 2014.
In May 2015 the Linux kernel introduced a deadlock bug via Commit b0c29f79ecea that caused a hang in user applications. The bug affected many enterprise Linux distributions, including 3.x and 4.x kernels, and Red Hat Enterprise Linux version 5, 6 and 7, SUSE Linux 12 and Amazon Linux.
Futexes have been implemented in OpenBSD since 2016.
Futexes have two basic operations,
- If the value stored at the address
val, puts the current thread to sleep.
- Wakes up
numnumber of threads waiting on the address
For more advanced uses there are number of other operations the most used being
WAKE_OP and both functions as more generic
WAKE operations. 
CMP_REQUEUE(old_addr, new_addr, num_wake, num_move, val)
- If the value stored at the address
num_wakethreads waiting on the address
old_addr, and enqueues
num_movethreads waiting on the address
old_addrto now wait on the address
new_addr. This can be used to avoid the thundering herd problem on wake.
WAKE_OP(addr1, addr2, num1, num2, op, op_arg, cmp, cmp_arg)
- Will read
op_argon it, and storing the result back to
addr2. Then it will wake
num1threads waiting on
addr1, and if the previously read value from
num2threads waiting on
addr2. This very flexible and generic wake is useful for implementation many synchronization primitives.
- Torvalds, Linus. "Futex Asynchronous Interface".
- "WaitOnAddress function". Retrieved 2019-11-01.
- "US8782674B2 Wait on address synchronization interface". Retrieved 2019-11-01.
- "[SECURITY] [DSA 2949-1] linux security update". Lists.debian.org. 2014-06-05. Retrieved 2014-06-08.
- "Linux futex_wait() bug..." 2015-05-13. Retrieved 2018-03-24.
- Mazurek, Michal. "'Futexes for OpenBSD' - MARC". marc.info. Retrieved 30 April 2017.
- "Zircon Kernel Concepts". fuchsia.dev. Retrieved 20 October 2019.
- "zx_futex_wait". fuchsia.dev. Retrieved 20 October 2019.
- Futexes Are Tricky Ulrich Drepper (Red Hat, v1.6, 2011)
- Linux futex(2) man page, FUTEX_CMP_REQUEUE section
- Zircon zx_futex_requeue documentation
- - futex() system call
- - futex semantics and usage
- Hubertus Franke, Rusty Russell, Matthew Kirkwood. Fuss, futexes and furwocks: Fast Userlevel Locking in Linux, Ottawa Linux Symposium 2002.
- Drepper, Ulrich (2011). "Futexes are Tricky" (PDF). 1.6. Red Hat.
- Bert Hubert (2004). Unofficial Futex manpages
- Ingo Molnar. "Robust Futexes", Linux Kernel Documentation
- "Priority Inheritance Futexes", Linux Kernel Documentation