Ubuntu Manpage: futex - fast user-space locking

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NAME

       futex - fast user-space locking

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). 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 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 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.

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 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

       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:

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

       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.

ERRORS

       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 reached.

       ENOSYS Invalid operation specified in op.

       ETIMEDOUT
              Timeout during the FUTEX_WAIT operation.

       EWOULDBLOCK
              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

       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

       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 user-space library referenced below.

COLOPHON

       This page is part of release 3.54 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/.