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       futex - Fast Userspace Locking


       #include <linux/futex.h>


       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
       development.  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
       semantics;  it  is  a  counter  that can be incremented and decremented
       atomically; processes can wait for the value to become positive.

       Futex operation is entirely userspace for the noncontended  case.   The
       kernel  is  only involved 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 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
       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.  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 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.  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
       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


       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
       abstraction  for  end-users.   Implementors are expected to be assembly
       literate and to have read the sources of the  futex  userspace  library
       referenced below.

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



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


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