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

NAME

       futex - Fast Userspace Locking

SYNOPSIS

       #include <linux/futex.h>

DESCRIPTION

       The  Linux  kernel  provides futexes ("Fast Userspace muTexes") as a building block for fast userspace 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  develop-
       ment.   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 semantics; 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 userspace for the noncontended case.  The kernel is only involved 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 only touched 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  multi-
       threaded.

   Semantics
       Any futex operation starts in userspace, but it may 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.  Afterwards,
       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.  Userspace 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 wait-
       ing.

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 onwards.

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 userspace 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

       futex(2)

       Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceedings of the	Ottawa	Linux  Symposium  2002),  futex  example  library,
       futex-*.tar.bz2 <URL:ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/>.

COLOPHON

       This  page is part of release 3.25 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								    2002-12-31								  FUTEX(7)