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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.   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.   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 onward.


       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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