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       futex - Fast Userspace Locking system call


       #include <linux/futex.h>

       #include <sys/time.h>

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


       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).  It is typically used to implement the
       contended case of a lock in shared memory, as described in futex(4).

       When a futex(4) 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(4).   As  these  semantics  involve  writing  non-portable
       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 parameter,
       along with a value val.

       Five operations are currently defined:

              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(4), 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.

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

              For  futex(4), 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).

              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.

       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, an error EAGAIN  is  returned.
              The argument timeout is ignored.


       Depending  on which operation was executed, the returned value can have
       differing meanings.

              Returns 0 if the process was woken by a FUTEX_WAKE call. In case
              of timeout, ETIMEDOUT is returned. If the futex was not equal to
              the expected value, the operation returns  EWOULDBLOCK.  Signals
              (or other spurious wakeups) cause FUTEX_WAIT to return EINTR.

              Returns the number of processes woken up.

              Returns the new file descriptor associated with the futex.

              Returns the number of processes woken up.

              Returns the number of processes woken up.


       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


       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.


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


       futex(4), ‘Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux’
       (proceedings  of  the  Ottawa  Linux  Symposium  2002),  futex  example
       library,                                                futex-*.tar.bz2