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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)
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