* call returns a pointer to the buffer from where such data shall be
* read, and its length is written in `*len`. Otherwise, `*len` is set
* to 0 and `NULL` is returned.
* call returns a pointer to the buffer from where such data shall be
* read, and its length is written in `*len`. Otherwise, `*len` is set
* to 0 and `NULL` is returned.