futex


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).
              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
       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.

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 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-08-29                          FUTEX(2)
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