NAME

       futex - fast user-space locking

SYNOPSIS

       #include <linux/futex.h>

DESCRIPTION

       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.

   Semantics
       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 waiting.

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 onward.

NOTES

       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.

SEE ALSO

       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  ⟨https://mirrors.kernel.org
       /pub/linux/kernel/people/rusty/⟩.