futex

FUTEX(7)                   Linux Programmer's Manual                  FUTEX(7)



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 ⟨ftp://ftp.kernel.org/pub/linux/kernel/people
       /rusty/⟩.

COLOPHON
       This page is part of release 5.03 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2017-09-15                          FUTEX(7)