FUTEX(7)                   Linux Programmer's Manual                  FUTEX(7)

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  seman-
       tics;  it  is  a counter that can be incremented and decremented atomi-
       cally; 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-contention, 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
       using  mmap(),  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 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 wait-
       ing.

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(7)