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/.