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       futex - fast user-space locking


       #include <linux/futex.h>


       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.

       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


       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 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.


       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  ⟨


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