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FUTEX(7)		   Linux Programmers Manual		     FUTEX(7)

       futex - Fast Userspace Locking


       The  Linux  kernel  provides  futexes  ("Fast  Userspace muTexes") as a
       building block for fast userspace locking and semaphores.  Futexes  are
       very  basic  and lend themselves well for building higher level locking
       abstractions such as POSIX mutexes.

       This page does not  set	out  to  document  all	design	decisions  but
       restricts  itself to issues relevant for application and library devel
       opment.	Most programmers will in fact not be  using  futexes  directly
       but  instead  rely  on system libraries built on them, such as the NPTL
       pthreads implementation.

       A futex is identified by a piece of memory which can be shared  between
       different  processes.   In  these different processes, it need not have
       identical addresses.  In its bare form, a futex	has  semaphore	seman
       tics;  it  is  a counter that can be incremented and decremented atomi
       cally; processes can wait for the value to become positive.

       Futex operation is entirely userspace for the non-contended case.   The
       kernel  is  only involved 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 only touched
       by atomic assembler instructions.  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 userspace, but it may necessary to commu
       nicate 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.  Afterwards,
       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 non-contended 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 wait
       ers.  Userspace 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 circum
       stances, 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 wait

       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 onwards.

       To reiterate, bare futexes are not intended as an easy to use  abstrac
       tion  for end-users.  Implementors are expected to be assembly literate
       and to have read the sources of the futex userspace library  referenced

       This  man  page	illustrates the most common use of the futex(2) primi
       tives: it is by no means the only one.


       Fuss, Futexes and Furwocks: Fast Userlevel Locking in  Linux  (proceed
       ings  of  the  Ottawa  Linux  Symposium	2002),	futex example library,
       futex-*.tar.bz2	      .

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2002-12-31			      FUTEX(7)

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