futex(7) — Linux manual page


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

NAME         top

       futex - fast user-space locking

SYNOPSIS         top

       #include <linux/futex.h>

DESCRIPTION         top

       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

       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.

       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         top

       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         top

       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         top

       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 

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2017-09-15                         FUTEX(7)

Pages that refer to this page: futex(2)pthreads(7)