ssize_t mongoc_gridfs_file_readv (mongoc_gridfs_file_t *file, mongoc_iovec_t *iov, size_t iovcnt, size_t min_bytes, uint32_t timeout_msec);
This function performs a scattered read from file, potentially blocking to read from the MongoDB server.
Timeout should be the number of milliseconds to allow blocking, or 0 to disallow blocking.
Returns the number of bytes read, or -1 on failure. Use mongoc_gridfs_file_error to retrieve error details.