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NAME

       futex - Fast Userspace Locking

SYNOPSIS

       #include <linux/futex.h>

DESCRIPTION

       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.

   Semantics
       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 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 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.

SEE ALSO

       futex(2)

       Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux  (proceedings  of  the  Ottawa
       Linux      Symposium      2002),      futex      example      library,     futex-*.tar.bz2
       <URL:ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/>.

COLOPHON

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