* allocated buffer or map it with mmap() when available.
* In case of success set *bufptr to the read or mmapped buffer, and
* *size to the size in bytes of the buffer in *bufptr.
+ * Unlike mmap this function succeeds with zero sized files, in this
+ * case *bufptr will be set to NULL and *size will be set to 0.
* The returned buffer must be released with av_file_unmap().
*
* @param log_offset loglevel offset used for logging