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NAME

       futex - Fast Userspace Locking system call

SYNOPSIS

       #include <linux/futex.h>
       #include <sys/time.h>

       int futex(int *uaddr, int op, int val, const struct timespec *timeout,
                 int *uaddr2, int val3);

DESCRIPTION

       The  futex()  system  call  provides a method for a program to wait for a value at a given
       address to change, and a method to wake up anyone waiting on a particular  address  (while
       the  addresses for the same memory in separate processes may not be equal, the kernel maps
       them internally so the same memory mapped  in  different  locations  will  correspond  for
       futex()  calls).   This system call is typically used to implement the contended case of a
       lock in shared memory, as described in futex(7).

       When a futex(7) operation did not finish uncontended in userspace, a call needs to be made
       to  the  kernel  to arbitrate.  Arbitration can either mean putting the calling process to
       sleep or, conversely, waking a waiting process.

       Callers of this function are expected to adhere to the semantics as set out  in  futex(7).
       As  these  semantics  involve  writing  nonportable  assembly  instructions,  this in turn
       probably means that most users will in fact be library authors and not general application
       developers.

       The  uaddr  argument  needs  to point to an aligned integer which stores the counter.  The
       operation to execute is passed via the op argument, along with a value val.

       Five operations are currently defined:

       FUTEX_WAIT
              This operation atomically verifies that the futex address uaddr still contains  the
              value  val,  and  sleeps awaiting FUTEX_WAKE on this futex address.  If the timeout
              argument is non-NULL, its contents describe the maximum duration of the wait, which
              is infinite otherwise.  The arguments uaddr2 and val3 are ignored.

              For futex(7), this call is executed if decrementing the count gave a negative value
              (indicating contention), and will sleep until another process  releases  the  futex
              and executes the FUTEX_WAKE operation.

       FUTEX_WAKE
              This  operation  wakes  at  most val processes waiting on this futex address (i.e.,
              inside FUTEX_WAIT).  The arguments timeout, uaddr2 and val3 are ignored.

              For futex(7), this is executed if incrementing the count  showed  that  there  were
              waiters, once the futex value has been set to 1 (indicating that it is available).

       FUTEX_FD (present up to and including Linux 2.6.25)
              To support asynchronous wakeups, this operation associates a file descriptor with a
              futex.  If another process executes a FUTEX_WAKE,  the  process  will  receive  the
              signal  number that was passed in val.  The calling process must close the returned
              file descriptor after use.  The arguments timeout, uaddr2 and val3 are ignored.

              To prevent race conditions, the caller should test if  the  futex  has  been  upped
              after FUTEX_FD returns.

              Because it was inherently racy, FUTEX_FD has been removed from Linux 2.6.26 onward.

       FUTEX_REQUEUE (since Linux 2.5.70)
              This  operation  was  introduced  in order to avoid a "thundering herd" effect when
              FUTEX_WAKE is used and all processes woken up need to acquire another futex.   This
              call wakes up val processes, and requeues all other waiters on the futex at address
              uaddr2.  The arguments timeout and val3 are ignored.

       FUTEX_CMP_REQUEUE (since Linux 2.6.7)
              There was a race in the intended use of  FUTEX_REQUEUE,  so  FUTEX_CMP_REQUEUE  was
              introduced.   This  is  similar  to  FUTEX_REQUEUE,  but  first  checks whether the
              location uaddr still contains the value val3.  If not, the operation fails with the
              error EAGAIN.  The argument timeout is ignored.

RETURN VALUE

       Depending  on  which  operation was executed, the returned value for a successful call can
       have differing meanings.

       FUTEX_WAIT
              Returns 0 if the process was woken by a FUTEX_WAKE call.  In case of  timeout,  the
              operation  fails  with  the  error  ETIMEDOUT.   If  the futex was not equal to the
              expected value, the operation fails  with  the  error  EWOULDBLOCK.   Signals  (see
              signal(7)) or other spurious wakeups cause FUTEX_WAIT to fail with the error EINTR.

       FUTEX_WAKE
              Returns the number of processes woken up.

       FUTEX_FD
              Returns the new file descriptor associated with the futex.

       FUTEX_REQUEUE
              Returns the number of processes woken up.

       FUTEX_CMP_REQUEUE
              Returns the number of processes woken up.

       In the event of an error, all operations return -1, and set errno to indicate the error.

ERRORS

       EACCES No read access to futex memory.

       EAGAIN FUTEX_CMP_REQUEUE  found  an  unexpected  futex  value.  (This probably indicates a
              race; use the safe FUTEX_WAKE now.)

       EFAULT Error in getting timeout information from userspace.

       EINVAL An operation was not defined or error in page alignment.

       ENFILE The system limit on the total number of open files has been reached.

       ENOSYS Invalid operation specified in op.

VERSIONS

       Initial futex support was merged in Linux 2.5.7 but with different semantics from what was
       described  above.   A 4-argument system call with the semantics described in this page was
       introduced in Linux 2.5.40.  In Linux 2.5.70 one argument was added.   In  Linux  2.6.7  a
       sixth argument was added—messy, especially on the s390 architecture.

CONFORMING TO

       This system call is Linux-specific.

NOTES

       To  reiterate,  bare futexes are not intended as an easy-to-use abstraction for end-users.
       (There is no wrapper function for this system call in glibc.)  Implementors  are  expected
       to  be  assembly  literate  and  to  have  read the sources of the futex userspace library
       referenced below.

SEE ALSO

       futex(7)

       Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux  (proceedings  of  the  Ottawa
       Linux Symposium 2002), online at
       http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf

       Futex example library, futex-*.tar.bz2 at
       ftp://ftp.nl.kernel.org/pub/linux/kernel/people/rusty/.

COLOPHON

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