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FUTEX(2)		   Linux Programmers Manual		     FUTEX(2)

       futex - Fast Userspace Locking system call


       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 mem
       ory in separate processes may not be equal, the kernel maps them inter
       nally  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  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  pro
	      cesses,  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 proba
	      bly 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 abstrac
       tion for end-users.  Implementors are expected to be assembly  literate
       and  to have read the sources of the futex userspace library referenced


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

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-01-31			      FUTEX(2)

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