CFNetwork (POST request) problem

I am writing a program witch would sent POST message to server. The body of the message has to be taken from NSString object. Everything was fine until I realized that CFReadStreamCreateForHTTPRequest function sends only half of the POST body (I was sniffing with wireshark). There was written in a description of that function to use CFReadStreamCreateForStreamedHTTPRequest instead in this situation, but I don't know how to do this. Please help me with a sample code.
Michael