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    SubjectRe: real POSIX.1b semaphores

    "Theodore Y. Ts'o" <tytso@MIT.EDU> writes:

    > True, we have to implement it. But we don't have to spend a lot of time
    > getting it super fast, or particularily elegant. I'd concetrate on
    > those parts of the POSIX.1b spec that are likely to be actually used by
    > real applications.....

    But I think that the named semaphores can be implemented on top of
    the unnamed semaphores. The kernel should keep a list of the names
    (the pseudo-filesystem) and return handle for a semaphore from the
    common pool.

    When I go back to my initial proposal:

    sem_init() returns simply the next free semaphore in the buffer


    sem_open() looks whether there is such a semaphore. If not and O_CREAT
    is given is returns the next free semaphore, i.e., it implicitly
    calls sem_init()

    The functions which use semaphores behave the same.

    (Please note that I don't talk anymore about a second pool for local
    semaphores. Xavier pointed out that this is really stupid since it
    would be slower than using a user-level only implementation.)

    One point still not clear is whether it's worth to create a
    pseudo filesystem for the named semaphores. I would vote for it
    and the standard author encourage this use. It would allow easy
    maintenance of the semaphore namespace. Keeping all names in a
    plane namespace (i.e., treating / as a normal character) would
    be simpler but once the semaphores are implemented I would think
    hundreds of semaphores used at the same time is easily possible.
    How do you like a directory with hundreds of entries? The POSIX
    people gave us this mean of structurization so me should use it.

    -- Uli
    --------------. ,-. Rubensstrasse 5
    Ulrich Drepper \ ,--------------------' \ 76149 Karlsruhe/Germany
    Cygnus Support `--' `------------------------

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