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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 mutexes, condition	variables,  read-write	locks,
       barriers, and semaphores.

       Most  programmers  will	in fact not be using futexes directly but will
       instead rely on system libraries built on  them,	 such  as  the	Native
       POSIX Thread Library (NPTL) (see pthreads(7)).

       A  futex is identified by a piece of memory which can be shared between
       processes or threads.  In these different processes, the futex 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	 occurs	 entirely  in  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 opti-
       mized for this situation.

       In its bare form, a futex is an aligned integer which is	 touched  only
       by  atomic  assembler instructions.  This integer is four bytes long on
       all platforms.  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.

       clone(2),     futex(2),	   get_robust_list(2),	   set_robust_list(2),
       set_tid_address(2), pthreads(7)

       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/>.

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Linux				  2015-12-28			      FUTEX(7)