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

NAME
       futex - fast user-space locking

SYNOPSIS
       #include	<linux/futex.h>
       #include	<sys/time.h>

       int futex(int *uaddr, int op, int val, const struct timespec *timeout,
		 int *uaddr2, int val3);
       Note: There is no glibc wrapper for this	system call; see NOTES.

DESCRIPTION
       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).  This system	call is	typically  used	 to  implement
       the  contended  case  of	 a  lock in shared memory, as described	in fu-
       tex(7).

       When a futex(7) operation did not finish	uncontended in user  space,  a
       call  needs to be made to the kernel to arbitrate.  Arbitration can ei-
       ther 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	assem-
       bly  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:

       FUTEX_WAIT
	      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	specify	the duration of	the wait.  (This interval will
	      be rounded up to the system clock	granularity, and kernel	sched-
	      uling  delays  mean  that	the blocking interval may overrun by a
	      small amount.)  If timeout is  NULL,  the	 call  blocks  indefi-
	      nitely.  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 FU-
	      TEX_WAKE operation.

       FUTEX_WAKE
	      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 FU-
	      TEX_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 onward.

       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 FU-
	      TEX_CMP_REQUEUE was introduced.  This is	similar	 to  FUTEX_RE-
	      QUEUE,  but  first  checks whether the location uaddr still con-
	      tains the	value val3.  If	not, the operation fails with the  er-
	      ror EAGAIN.  The argument	timeout	is ignored.

RETURN VALUE
       In  the	event  of an error, all	operations return -1, and set errno to
       indicate	the error.  The	return value on	success	depends	on the	opera-
       tion, as	described in the following list:

       FUTEX_WAIT
	      Returns  0  if  the process was woken by a FUTEX_WAKE call.  See
	      ERRORS for the various possible error returns.

       FUTEX_WAKE
	      Returns the number of processes woken up.

       FUTEX_FD
	      Returns the new file descriptor associated with the futex.

       FUTEX_REQUEUE
	      Returns the number of processes woken up.

       FUTEX_CMP_REQUEUE
	      Returns the number of processes woken up.

ERRORS
       EACCES No read access to	futex memory.

       EAGAIN FUTEX_CMP_REQUEUE	detected that the value	pointed	to by uaddr is
	      not  equal to the	expected value val3.  (This probably indicates
	      a	race; use the safe FUTEX_WAKE now.)

       EFAULT Error retrieving timeout information from	user space.

       EINTR  A	FUTEX_WAIT operation was interrupted by	 a  signal  (see  sig-
	      nal(7)) or a spurious wakeup.

       EINVAL Invalid argument.

       ENFILE The  system  limit  on  the  total number	of open	files has been
	      reached.

       ENOSYS Invalid operation	specified in op.

       ETIMEDOUT
	      Timeout during the FUTEX_WAIT operation.

       EWOULDBLOCK
	      op was FUTEX_WAIT	and the	value pointed  to  by  uaddr  was  not
	      equal to the expected value val at the time of the call.

VERSIONS
       Initial	futex support was merged in Linux 2.5.7	but with different se-
       mantics from what was described above.  A 4-argument system  call  with
       the  semantics  described  in this page was introduced in Linux 2.5.40.
       In Linux	2.5.70,	one argument was added.	 In Linux 2.6.7, a sixth argu-
       ment was	added--messy, especially on the	s390 architecture.

CONFORMING TO
       This system call	is Linux-specific.

NOTES
       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 user-space library referenced	below.

SEE ALSO
       restart_syscall(2), futex(7)

       Fuss, Futexes and Furwocks: Fast	Userlevel Locking in  Linux  (proceed-
       ings of the Ottawa Linux	Symposium 2002), online	at
       <http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf>

       Futex example library, futex-*.tar.bz2 at
       <ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/>

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-05-21			      FUTEX(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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