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

NAME         top

       futex - fast user-space locking

SYNOPSIS         top

       #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.

DESCRIPTION         top

       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 probably 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 interval 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 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
              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         top

       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 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.

ERRORS         top

       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
              signal(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.

VERSIONS         top

       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.

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       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
       referenced below.

SEE ALSO         top

       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

COLOPHON         top

       This page is part of release 4.02 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)