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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 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:

              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).

       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 onwards.

       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.


       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, 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

              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-argument system call with
       the  semantics  given  here  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.


       This system call is Linux-specific.


       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.



       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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