How to change the working directory in iPhone programatlically(FTP)

Question

I am trying to establish FTP connection with FTP server in iOS (Authentication based). To maintain the login status I am listing the directory and if the directory listing count is zero then login is unsuccessful.

Now my problem is whenever I log in then the root directory is "romfs" as shown in screen shot in red circle and I want the directory access to other directory which is at same level as highlighted in red rectangle. So i directly set the url to this directory with the proper username/ password. like this

    ----"ftp://server_ip/ahci02:2/db/" 

But it is trying to list \romfs folder So my question is how to change the working directory from "romfs" to "ahci02:2" programmatically in iOS.

Thnx in advance.

Pgggyyyyy