FUTEX(7)                  (2017-09-15)                   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 non-
          contended case.  The kernel is involved only to arbitrate
          the contended case.  As any sane design will strive for non-
          contention, 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 applica-
          tion is commonly called multithreaded.

        Semantics
          Any futex operation starts in user space, but it may be nec-
          essary 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.

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     FUTEX(7)                  (2017-09-15)                   FUTEX(7)

          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
          .}f (proceedings of the Ottawa Linux Symposium 2002), futex
          example library, futex-*.tar.bz2

     COLOPHON
          This page is part of release 5.10 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/.

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