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