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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 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).   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 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 assembly instructions, this in turn proba‐
       bly means that most users will in fact be library  authors  and  not  general  application

       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  inter‐
              val  will  be  rounded  up  to  the system clock granularity, and kernel scheduling
              delays mean that the blocking interval may overrun by a small amount.)  If  timeout
              is NULL, the call blocks indefinitely.  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 sig‐
              nal  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 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 loca‐
              tion 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 operation, as described in the following list:

              Returns 0 if the process was woken by a FUTEX_WAKE call.  See ERRORS for the  vari‐
              ous 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

       EFAULT Error retrieving timeout information from user space.

       EINTR  A  FUTEX_WAIT  operation  was interrupted by a signal (see signal(7)) or a spurious

       EINVAL Invalid argument.

       ENFILE The system limit on the total number of open files has been reached.

       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 argument 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 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 user-space library  ref‐
       erenced below.

       restart_syscall(2), futex(7)

       Fuss,  Futexes  and  Furwocks:  Fast Userlevel Locking in Linux (proceedings 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-05-21                                   FUTEX(2)

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