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FUTEX(2)		   Linux Programmer's Manual		      FUTEX(2)

       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 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.  (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 (proceed‐
       ings of the  Ottawa  Linux  Symposium  2002),  futex  example  library,
       futex-*.tar.bz2	    <URL:ftp://ftp.nl.kernel.org/pub/linux/kernel/peo‐

       This page is part of release 3.22 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				  2008-11-27			      FUTEX(2)

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