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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(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 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(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.

              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).

              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 is inherently racy, FUTEX_FD is scheduled for removal
              in June 2007; any applications that use it should be fixed  now.

       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 for a
       successful call 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 fails with the  error
              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.

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


       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

       ENOSYS Invalid operation specified in op.


       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.


       This system call is Linux-specific.


       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.



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


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