phpman > man > futex(7)

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



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  in‐
       volved  only  to  arbitrate  the  contended case.  As any sane design will strive for noncon‐
       tention, futexes are also optimized for this situation.

       In its bare form, a futex is an aligned integer which is touched only by atomic assembler in‐
       structions.   This integer is four bytes long on all platforms.  Processes can share this in‐
       teger 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 nega‐
       tive 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  uncon‐
       tended.   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 pro‐
       grammer  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  de‐
       scribed 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.  Im‐
       plementors 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  ⟨ftp://ftp.kernel.org/pub/linux
       /kernel/people/rusty/⟩.

COLOPHON
       This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.



Linux                                        2017-09-15                                     FUTEX(7)