Futex
In computing, a futex 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 kernel-space wait queue that is attached to an atomic integer in userspace. Multiple processes or threads operate on the integer entirely in userspace, and only resort to the fast but still more 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 has contention; since most operations do not require arbitration between processes, this will not happen in most cases.
History
Hubertus Franke, Matthew Kirkwood, Ingo Molnár, and Rusty Russell originated the futex mechanism on Linux in 2002.In the same year, 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 then 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 kernel mainline">Kernel (operating system)">kernel mainline since the December 2003 release of 2.6.x stable kernel series.
Futex functionality has been implemented in Microsoft Windows since Windows 8 or Windows Server 2012 under the name WaitOnAddress.
In 2013, Microsoft patented futex-related WaitOnAddress 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.
Apple implemented futex in iOS/iPadOS/tvOS 17.4, macOS 14.4, watchOS 10.4 and visionOS 1.1.
Futex like functionality was added to C++ with the
atomic::wait, atomic::notify_one, and atomic::notify_all operations in C++20.Support for FUTEX2 in the Linux kernel was designed to support two new main features, first something that can be used to implement the Win32 API WaitForMultipleObjects, and second to be able to wait on addresses other than 32bit ones. The first step was integrated in 5.16 in November 2021. with the waitv syscall.
Operations
Futexes have two basic operations,WAIT and WAKE.WAIT, WAKEFor more advanced uses, there are a number of other operations, the most used being
CMP_REQUEUE and WAKE_OP, which both function as more generic WAKE operations.CMP_REQUEUE, WAKE_OPFUTEX2 API:
WAITV