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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);

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
       futex(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
       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 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
	      FUTEX_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
	      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 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
	      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.

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
       semantics 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.nl.kernel.org/pub/linux/kernel/people/rusty/⟩

COLOPHON
       This  page  is  part of release 3.54 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				  2013-09-04			      FUTEX(2)
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