futex man page on Debian

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

NAME
       futex - Fast Userspace Locking system call

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	 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 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	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.

       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 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.

RETURN VALUE
       Depending on which operation was executed, the  returned	 value	for  a
       successful call can have differing meanings.

       FUTEX_WAIT
	      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
	      EINTR.

       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.

       In the event of an error, all operations return -1, and	set  errno  to
       indicate the error.

ERRORS
       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
	      reached.

       ENOSYS Invalid operation specified in op.

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 userspace library referenced below.

SEE ALSO
       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 3.27 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				  2010-08-29			      FUTEX(2)
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