File content as of revision 2:4a51bdd24745:

#include "mbed.h"
#include "EthernetInterface.h"
#include <stdio.h>
#include <string>

using namespace     std;

#define PORT        80

EthernetInterface   ethernet;

TCPSocketServer     server;
bool                serverIsListening = false;

TCPSocketConnection client;
bool                clientIsConnected = false;

const int           OFF = 0;
const int           ON = 1;

DigitalOut          sw(LED1);
float               roomTemp = 21.8;    // A temperature sensor output

const string        PASSWORD = "secret";    // change as you like
const string        HTTP_OK = "HTTP/1.0 200 OK";
const string        MOVED_PERM = "HTTP/1.0 301 Moved Permanently\r\nLocation: ";
const string        UNAUTHORIZED = "HTTP/1.0 401 Unauthorized";
string              httpHeader;     // HTTP header
string              httpContent;    // HTTP content

/**
 * @brief   Defines a custom MAC address
 * @note    Uncomment the code below to define a unique MAC address.
 *          Modify the mac array items as needed.
 * @param
 * @retval
 */
//extern "C" void mbed_mac_address(char* mac) {
//    mac[0] = 0x00;
//    mac[1] = 0x01;
//    mac[2] = 0x02;
//    mac[3] = 0x03;
//    mac[4] = 0x04;
//    mac[5] = 0x05;
//};

/**
 * @brief   Analyses the received URL
 * @note    The string passed to this function will look like this:
 *          GET /password HTTP/1.....
 *          GET /password/ HTTP/1.....
 *          GET /password/?sw=1 HTTP/1.....
 *          GET /password/?sw=0 HTTP/1.....
 * @param   url URL string
 * @retval -1 invalid password
 *         -2 no command given but password valid
 *         -3 just refresh page
 *          0 switch off
 *          1 switch on
 */
int8_t analyseURL(string& url) {
    if(url.substr(5, PASSWORD.size()) != PASSWORD)
        return(-1);

    uint8_t pos = 5 + PASSWORD.size();

    if(url.substr(pos, 1) == " ")
        return(-2);

    if(url.substr(pos++, 1) != "/")
        return(-1);

    string  cmd(url.substr(pos, 5));

    if(cmd == "?sw=0")
        return(OFF);

    if(cmd == "?sw=1")
        return(ON);

    return(-3);
}

/**
 * @brief
 * @note
 * @param
 * @retval
 */
string& movedPermanently(uint8_t flag) {
    if(flag == 1)
        httpContent = "/" + PASSWORD + "/";
    else
        httpContent = "";

    httpContent += "<h1>301 Moved Permanently</h1>\r\n";

    return(httpContent);
}

/**
 * @brief
 * @note
 * @param
 * @retval
 */
string& showWebPage(uint8_t status) {
    char roomTempStr[5];

    //roomTemp = ds1820.read();
    sprintf(roomTempStr, "%3.1f", roomTemp);

    httpContent = "<h2><a href=\".\" title=\"Click to refresh the page\">Smart Home</a></h2>"; 
    httpContent += "<pre>Temperature:\t" + string(roomTempStr) + "&deg;C\r\n</pre>";

    if(status == ON) {
        httpContent += "<pre>\r\nHeating:\t<font color=#FF0000>On </font>";
        httpContent += " <a href=\"./?sw=0\"><button>Turn off</button></a>\r\n";
    }
    else {
        httpContent += "<pre>\r\nHeating:\t<font color=#999999>Off</font>";
        httpContent += " <a href=\"./?sw=1\"><button>Turn on</button></a>\r\n";
    }

    httpContent += "</pre>\r\n";
    httpContent += "<hr>\r\n";
    httpContent += "<pre>2017 ARMmbed</pre>";
    return httpContent;
}

/**
 * @brief
 * @note
 * @param
 * @retval
 */
void sendHTTP(TCPSocketConnection& client, string& header, string& content) {
    char    contentLength[5] = { };

    header += "\r\nContent-Type: text/html\r\n";
    header += "Content-Length: ";
    sprintf(contentLength, "%d", content.length());
    header += string(contentLength) + "\r\n";
    header += "Pragma: no-cache\r\n";
    header += "Connection: About to close\r\n";
    header += "\r\n";

    string  webpage = header + content;
    client.send((char*)webpage.c_str(), webpage.length());
    clientIsConnected = false;
    printf("HTTP sent.\n\r");
}

/**
 * @brief
 * @note
 * @param
 * @retval
 */
int main(void) {

    //setup ethernet interface
    ethernet.init(); //Use DHCP
    //ethernet.init("192.168.1.181", "255.255.255.0", "192.168.1.1");   // Use static IP
    ethernet.connect();
    printf("USAGE: Type '%s/%s/' into your web browser and hit ENTER\r\n", ethernet.getIPAddress(), PASSWORD.c_str());
    printf("NOTE:  Don't forget to type the last '/'.\r\n\r\n");

    //setup tcp socket
    if(server.bind(PORT) < 0) {
        printf("HTTP server bind failed.\n\r");
        return -1;
    }
    else {
        printf("HTTP server bind succeeded.\n\r");
        serverIsListening = true;
    }

    if(server.listen(1) < 0) {
        printf("HTTP server listen failed.\n\r");
        return -1;
    }
    else {
        printf("HTTP server is listening...\n\r");
    }

    //listening for http GET request
    while(serverIsListening) {

        if(server.accept(client) < 0) {
            printf("Failed to accept connection.\n\r");
        }
        else {
            printf("\r\n=========================================\r\n");
            printf("Connection accepted!\n\rIP: %s\n\r", client.get_address());
            clientIsConnected = true;

            while(clientIsConnected) {
                char    buffer[1024] = { };
                switch(client.receive(buffer, 1023)) {
                case 0:
                    printf("Recieved buffer is empty.\n\r");
                    clientIsConnected = false;
                    break;

                case -1:
                    printf("Failed to read data from client.\n\r");
                    clientIsConnected = false;
                    break;

                default:
                    printf("Recieved Data: %d\n\r\n\r%.*s\n\r", strlen(buffer), strlen(buffer), buffer);

                    string  received(buffer);
                    if(received.substr(0, 3) != "GET") {
                        httpHeader = HTTP_OK;
                        httpContent = "<h1>200 OK</h1>";
                        sendHTTP(client, httpHeader, httpContent);
                        continue;
                    }

                    if(received.substr(0, 6) == "GET / ") {
                        httpHeader = HTTP_OK;
                        httpContent = "<p>USAGE: Type 'http://ip_address/password/' into your web browser and hit ENTER</p>\r\n";
                        sendHTTP(client, httpHeader, httpContent);
                        continue;
                    }

                    int cmd = analyseURL(received);

                    if(cmd == -2) {

                        // redirect to the correct base URL
                        httpHeader = MOVED_PERM;
                        sendHTTP(client, httpHeader, movedPermanently(1));
                        continue;
                    }

                    if(cmd == -1) {
                        httpHeader = UNAUTHORIZED;
                        httpContent = "<h1>401 Unauthorized</h1>";
                        sendHTTP(client, httpHeader, httpContent);
                        continue;
                    }

                    if(cmd == ON) {
                        sw = ON;    // turn the switch on
                    }

                    if(cmd == OFF) {
                        sw = OFF;   // turn the switch off
                    }

                    httpHeader = HTTP_OK;
                    sendHTTP(client, httpHeader, showWebPage(sw));
                    break;
                }
            }

            printf("Closing connection.\n\r");
            printf("HTTP server is listening...\n\r");
            client.close();
        }
    }
}