it is an empty area,what empty area contains? or is it pointing to null?
When you create a DMA channel by using CyU3PDmaChannelCreate, it will return the result. Once success, you will get a block of memory which is the DMA buffer. Every buffer created in the System memory has a descriptor associated with it.
In the DMA descriptor structure, the filed uint8_t *buffer is the pointer to buffer which will be used.
It is just like the case when you invoke malloc API in C language.