In computing, a futex is a kernelsystem 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 kernelspacewait queue that is attached to an atomicinteger in userspace. Multiple processes or threads operate on the integer entirely in userspace, and only resort to relatively expensive system calls to request operations 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.
History
On Linux, Hubertus Franke, Matthew Kirkwood, Ingo Molnár and Rusty Russell 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 /dev or /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 2014 the CVE system announced a vulnerability discovered in the Linux kernel's futex subsystem that allowed denial-of-service attacks or local privilege escalation. In May 2015 the Linux kernel introduced a deadlock bug via 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. The futex mechanism is one of the core concepts of the Zircon kernel in Google's Fuchsia operating system since at least April 2018.
Operations
Futexes have two basic operations, WAIT and WAKE. A third operation called REQUEUE is available and functions as a more generic WAKE operation that can move threads between waiting queues.
