readahead reads count pages in the page cache. If a page is not in the page cache, it is added to the page cache.
fd is the file descriptor of the file being read. offset is the location within the file to start reading. A page in the page cache is identified by a files inode and by the offset within the file. fd and offset are used to identify the page in the page cache.
The offset is incremented by one to read the next page until count pages are read. If count pages is larger than the size of the file, then only up to the size of the file is read.
readahead returns 0 on success; otherwise, it returns the error listed in the "Errors" section. If the offset specified is greater than the size of the file, then nothing is read and 0 is returned.