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



NAME
       futex - Fast Userspace Locking system call

SYNOPSIS
       #include <&lt;linux/futex.h>&gt;
       #include <&lt;sys/time.h>&gt;

       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).  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:

       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 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.

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.  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),  futex  example  library,
       futex-*.tar.bz2      <URL:ftp://ftp.nl.kernel.org/pub/linux/kernel/peo-
       ple/rusty/>.

COLOPHON
       This page is part of release 3.05 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                             2008-01-31                          FUTEX(2)