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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 development.  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  pro‐
       cesses.   In these different processes, it need not have identical addresses.  In its bare
       form, a futex has semaphore semantics; it is a counter that can be incremented and  decre‐
       mented atomically; 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 mul‐

       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  opera‐

       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.

       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.


       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

       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2012-08-05                                   FUTEX(7)

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