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FUTEX(4)                   Linux Programmer's Manual                  FUTEX(4)



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 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 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 non-contended  case.  The
       kernel  is  only  involved to arbitrate the contended case. As any sane
       design will strive for non-contension, futexes are also  optimised  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 over
       mmap,  via shared 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 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 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.

NOTES
       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
       below.

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

AUTHORS
       Futexes were designed and worked on by Hubertus Franke (IBM  Thomas  J.
       Watson  Research  Center),  Matthew Kirkwood, Ingo Molnar (Red Hat) and
       Rusty Russell (IBM Linux Technology Center). This page written by  bert
       hubert.

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 onwards.

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/ker-
       nel/people/rusty/>.



                                  2002-12-31                          FUTEX(4)