+ // do read up to size bytes of data into the provided buffer
+ //
+ // this method should return 0 if EOF has been reached or an error occurred
+ // (m_lasterror should be set accordingly as well) or the number of bytes
+ // read
+ virtual size_t OnSysRead(void *buffer, size_t size) = 0;
+
+ // write-back buffer support
+ // -------------------------
+
+ // return the pointer to a buffer big enough to hold sizeNeeded bytes
+ char *AllocSpaceWBack(size_t sizeNeeded);
+
+ // read up to size data from the write back buffer, return the number of
+ // bytes read
+ size_t GetWBack(void *buf, size_t size);
+
+ // write back buffer or NULL if none