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

       futex - fast user-space locking

       #include <linux/futex.h>
       #include <sys/time.h>

       int futex(int *uaddr, int op, int val, const struct timespec *timeout,
                 int *uaddr2, int val3);
       Note: There is no glibc wrapper for this system call; see NOTES.

       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

       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:

              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.

              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.

       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:

              Returns  0  if  the process was woken by a FUTEX_WAKE call.  See
              ERRORS for the various possible error returns.

              Returns the number of processes woken up.

              Returns the new file descriptor associated with the futex.

              Returns the number of processes woken up.

              Returns the number of processes woken up.

       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

       ENOSYS Invalid operation specified in op.

              Timeout during the FUTEX_WAIT operation.

              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.

       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.

       This system call is Linux-specific.

       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.

       restart_syscall(2), futex(7)

       Fuss, Futexes and Furwocks: Fast Userlevel Locking in  Linux  (proceed-
       ings of the Ottawa Linux Symposium 2002), online at

       Futex example library, futex-*.tar.bz2 at

       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

Linux                             2014-05-21                          FUTEX(2)

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