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
   optimized 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
   waiters.   User  space 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
   circumstances,  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


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

   This  man  page  illustrates  the  most  common  use  of  the  futex(2)
   primitives; 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
   (proceedings  of  the  Ottawa  Linux  Symposium  2002),  futex  example
   library, futex-*.tar.bz2  ⟨ftp://ftp.kernel.org/pub/linux/kernel/people


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at


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