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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 memory  in  separate
       processes  may  not  be equal, the kernel maps them internally 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 con-
              tention), 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 processes, 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  mean-
       ings.

       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  probably  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 argu-
       ment 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 abstraction 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 (proceedings of the Ottawa Linux Symposium 2002),
       futex example library, futex-*.tar.bz2 <URL:ftp://ftp.nl.kernel.org/pub/linux/kernel/people/rusty/>;.

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-27                          FUTEX(2)