Futex: Meaning (information, definition, explanation, facts)

A futex (short for "fast userspace mutex") is a basic tool to realize locking and building higher-level locking abstractions such as semaphores and POSIX mutexes on Linux. They first appeared in the development kernel version 2.5.7; the semantics stabilized as of version 2.5.40, and they are present in the 2.6.x stable kernel series.

Futexes were created by Hubertus Franke (IBM Thomas J. Watson Research Center), Matthew Kirkwood, Ingo Molnar (Red Hat) and Rusty Russell (IBM Linux Technology Center).

Basically, a futex consists of a piece of memory (an aligned integer) that can be shared between processes; it can be incremented and decremented by atomic assembler instructions, and processes can wait for the value to become positive. Futex operation is done almost entirely in userspace; the kernel is only involved when a contended case needs to be arbitrated.

References

• Rusty Russell, fuss, futexes and furwocks: Fast Userlevel Locking in Linux, Ottawa Linux Symposium 2002, online