NAME
        LockIBase -- Invoke semaphore arbitration of intuitionbase.

    SYNOPSIS
        lock = lockibase( locknumber )
        D0                D0

        ULONG LockIBase( ULONG );

    FUNCTION
        Grabs Intuition internal semaphore so that caller may examine
        intuitionbase safely.  this function is not a magic "fix all my
        race conditions" panacea.

        The idea here is that you can get the locks Intuition needs before
        such intuitionbase fields as activewindow and firstscreen are
        changed, or linked lists of windows and screens are changed.

        Do Not Get Tricky with this entry point, and do not hold these locks
        for long, as all Intuition input processing will wait for you to
        surrender the lock by a call to unlockibase().

        NOTE WELL: A call to this function MUST be paired with a subsequent
        call to unlockibase(), and soon, please.

        NOTE WELL: Do not call any Intuition functions (nor any graphics,
        layers, dos, or other high-level system function) while
        holding this lock.

    INPUTS
        A long unsigned integer, LockNumber, specifies which of Intuition's
        internal locks you want to get.  This parameter should be zero for all
        forseeable uses of this function, which will let you examine active
        fields and linked lists of screens and windows with safety.

    RESULT
        Returns another ULONG which should be passed to unlockibase() to
        surrender the lock gotten by this call.

    BUGS
        This function must not be called while holding any other system locks
        such as layer or LayerInfo locks.

    SEE ALSO
        unlockibase(), layers.library/locklayerinfo(),
        exec.library/obtainsemaphore()