returns the pointer to the beginning of the contiguous block of
empty space within the virtual ring buffer where data may be placed.
The pointer has meaning only if there is space in the buffer.
If there is no space in the buffer, an arbitrary pointer is
returned that must not be used.
to determine if
has a meaningful pointer.
The caller may access all of this space, and should call
to indicate how much data has been placed into
specifies the virtual ring buffer to obtain the empty space pointer from.
If a NULL or invalid pointer is given, results are unpredictable
and are likely to be disasterous.
The pointer to the empty space in the virtual ring buffer is returned.