futex(7) — Linux manual page

futex(7)            Miscellaneous Information 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 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         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 
       ⟨https://mirrors.kernel.org/pub/linux/kernel/people/rusty/⟩.

Linux man-pages (unreleased)     (date)                         futex(7)

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

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