Write full HTTP POST request packet - web-services

Question

I need to write a full http request for invoke a SOAP service. I don't have libraries for soap request so i need to write full HTTP packet. this is how i've proceeded (i'm programming an arduino board):
String body = HttpRequestBody("33", "77%");
client.println("POST /dataserver HTTP/1.1");
client.println("Accept: text/xml, multipart/related");
client.println("Content-Type: text/xml; charset=utf-8");
client.println("SOAPAction: \"http://example.com/Functions/SendDataRequest\"");
client.println("User-Agent: Arduino WiFiShield");
client.println("Content-Length: "+body.length());
client.println("Host: arduino-data-server.appspot.com/dataserver");
client.println("Connection: Keep-Alive");
client.println();
client.println(body);
client represent connection to my webservice. This is HttpRequestBody function:
String HttpRequestBody(String v1, String v2) {
Serial.println("Generating xml message...");
String res = "";
res += "<?xml version=\"1.0\"?>\n\r";
res +="<S:Envelope xmlns:S=\"http://schemas.xmlsoap.org/soap/envelope/\"\n\r";
res +="<S:Body>\n\r";
res +="<ns2:sendData xmlsn:ns2=\"http://example.com\">\n\r";
res +="<arg0>"+v1+"</arg0>\n\r";
res +="<arg1>"+v2+"</arg1>\n\r";
res +="</ns2:sendData>\n\r";
res +="</S:Body>\n\r";
res +="</S:Envelope>\n\r";
Serial.println(res);
return res;
}
But something gone wrong and i can't contact webserver. Webserver works and it's reacable, because if i change POST into GET, on webservice log, i see connection. How can i fix it?

Answer 1

In HttpRequestBody you are allocating: String res = ""; and then updating it. Finally, you return res. But, res was allocated on the stack of HttpRequestBody (??), you are NOT guaranteed that it will be there after HttpRequestbody terminates.
You probably need to do the C++ equivalent of malloc used in C code, in order to ensure that res is on the heap and does not get freed.