man > futex(7)

futex(7)                          Miscellaneous Information 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  https://mirrors.kernel.org/pub/linux
       /kernel/people/rusty/.

Linux man-pages 6.7                          2023-10-31                                     futex(7)