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

       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 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 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.  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	 wait‐
       ers.   User space should now set the counter to 1 and instruct the ker‐
       nel 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(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 wait‐

       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 abstrac‐
       tion for end-users.  Implementors are expected to be assembly  literate
       and to have read the sources of the futex user-space library referenced

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


       Fuss,  Futexes  and Furwocks: Fast Userlevel Locking in Linux (proceed‐
       ings of the  Ottawa  Linux  Symposium  2002),  futex  example  library,
       futex-*.tar.bz2 ⟨ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/⟩.

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2012-08-05			      FUTEX(7)

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