Fix UdpClient semantics, add NTPClient example

UdpClient used to create a new socket for each begin/beginPacket call. This made bidirectional communication impossible.
Fix #64, fix #53.

Author: igrr

README.md

@@ -79,7 +79,7 @@ This is mostly similar to WiFi shield library. Differences include:
 - ```WiFi.printDiag(Serial);``` will print out some diagnostic info
 
 WiFiServer, WiFiClient, and WiFiUDP behave mostly the same way as with WiFi shield library.
-Three samples are provided for this library.
+Four samples are provided for this library.
 
 #### Ticker ####
 

hardware/esp8266com/esp8266/libraries/ESP8266WiFi/examples/NTPClient/NTPClient.ino

@@ -0,0 +1,146 @@
+/*
+
+ Udp NTP Client
+
+ Get the time from a Network Time Protocol (NTP) time server
+ Demonstrates use of UDP sendPacket and ReceivePacket
+ For more on NTP time servers and the messages needed to communicate with them,
+ see http://en.wikipedia.org/wiki/Network_Time_Protocol
+
+ created 4 Sep 2010
+ by Michael Margolis
+ modified 9 Apr 2012
+ by Tom Igoe
+ updated for the ESP8266 12 Apr 2015 
+ by Ivan Grokhotkov
+
+ This code is in the public domain.
+
+ */
+
+#include <ESP8266WiFi.h>
+#include <WiFiUdp.h>
+
+char ssid[] = "*************";  //  your network SSID (name)
+char pass[] = "********";       // your network password
+
+
+unsigned int localPort = 2390;      // local port to listen for UDP packets
+
+IPAddress timeServer(129, 6, 15, 28); // time.nist.gov NTP server
+
+const int NTP_PACKET_SIZE = 48; // NTP time stamp is in the first 48 bytes of the message
+
+byte packetBuffer[ NTP_PACKET_SIZE]; //buffer to hold incoming and outgoing packets
+
+// A UDP instance to let us send and receive packets over UDP
+WiFiUDP udp;
+
+void setup()
+{
+  Serial.begin(115200);
+  Serial.println();
+  Serial.println();
+
+  // We start by connecting to a WiFi network
+  Serial.print("Connecting to ");
+  Serial.println(ssid);
+  WiFi.begin(ssid, pass);
+  
+  while (WiFi.status() != WL_CONNECTED) {
+    delay(500);
+    Serial.print(".");
+  }
+  Serial.println("");
+  
+  Serial.println("WiFi connected");
+  Serial.println("IP address: ");
+  Serial.println(WiFi.localIP());
+
+  Serial.println("Starting UDP");
+  udp.begin(localPort);
+  Serial.print("Local port: ");
+  Serial.println(udp.localPort());
+}
+
+void loop()
+{
+  sendNTPpacket(timeServer); // send an NTP packet to a time server
+  // wait to see if a reply is available
+  delay(1000);
+  
+  int cb = udp.parsePacket();
+  if (!cb) {
+    Serial.println("no packet yet");
+  }
+  else {
+    Serial.print("packet received, length=");
+    Serial.println(cb);
+    // We've received a packet, read the data from it
+    udp.read(packetBuffer, NTP_PACKET_SIZE); // read the packet into the buffer
+
+    //the timestamp starts at byte 40 of the received packet and is four bytes,
+    // or two words, long. First, esxtract the two words:
+
+    unsigned long highWord = word(packetBuffer[40], packetBuffer[41]);
+    unsigned long lowWord = word(packetBuffer[42], packetBuffer[43]);
+    // combine the four bytes (two words) into a long integer
+    // this is NTP time (seconds since Jan 1 1900):
+    unsigned long secsSince1900 = highWord << 16 | lowWord;
+    Serial.print("Seconds since Jan 1 1900 = " );
+    Serial.println(secsSince1900);
+
+    // now convert NTP time into everyday time:
+    Serial.print("Unix time = ");
+    // Unix time starts on Jan 1 1970. In seconds, that's 2208988800:
+    const unsigned long seventyYears = 2208988800UL;
+    // subtract seventy years:
+    unsigned long epoch = secsSince1900 - seventyYears;
+    // print Unix time:
+    Serial.println(epoch);
+
+
+    // print the hour, minute and second:
+    Serial.print("The UTC time is ");       // UTC is the time at Greenwich Meridian (GMT)
+    Serial.print((epoch  % 86400L) / 3600); // print the hour (86400 equals secs per day)
+    Serial.print(':');
+    if ( ((epoch % 3600) / 60) < 10 ) {
+      // In the first 10 minutes of each hour, we'll want a leading '0'
+      Serial.print('0');
+    }
+    Serial.print((epoch  % 3600) / 60); // print the minute (3600 equals secs per minute)
+    Serial.print(':');
+    if ( (epoch % 60) < 10 ) {
+      // In the first 10 seconds of each minute, we'll want a leading '0'
+      Serial.print('0');
+    }
+    Serial.println(epoch % 60); // print the second
+  }
+  // wait ten seconds before asking for the time again
+  delay(10000);
+}
+
+// send an NTP request to the time server at the given address
+unsigned long sendNTPpacket(IPAddress& address)
+{
+  Serial.println("sending NTP packet...");
+  // set all bytes in the buffer to 0
+  memset(packetBuffer, 0, NTP_PACKET_SIZE);
+  // Initialize values needed to form NTP request
+  // (see URL above for details on the packets)
+  packetBuffer[0] = 0b11100011;   // LI, Version, Mode
+  packetBuffer[1] = 0;     // Stratum, or type of clock
+  packetBuffer[2] = 6;     // Polling Interval
+  packetBuffer[3] = 0xEC;  // Peer Clock Precision
+  // 8 bytes of zero for Root Delay & Root Dispersion
+  packetBuffer[12]  = 49;
+  packetBuffer[13]  = 0x4E;
+  packetBuffer[14]  = 49;
+  packetBuffer[15]  = 52;
+
+  // all NTP fields have been given values, now
+  // you can send a packet requesting a timestamp:
+  udp.beginPacket(address, 123); //NTP requests are to port 123
+  udp.write(packetBuffer, NTP_PACKET_SIZE);
+  udp.endPacket();
+}

hardware/esp8266com/esp8266/libraries/ESP8266WiFi/src/WiFiUdp.cpp

@@ -66,10 +66,10 @@ WiFiUDP::~WiFiUDP()
 /* Start WiFiUDP socket, listening at local port */
 uint8_t WiFiUDP::begin(uint16_t port) 
 {
-    if (_ctx)
-    {
+    if (_ctx) {
         _ctx->unref();
     }
+
     _ctx = new UdpContext;
     ip_addr_t addr;
     addr.addr = INADDR_ANY;
@@ -78,13 +78,11 @@ uint8_t WiFiUDP::begin(uint16_t port)
 
 uint8_t WiFiUDP::beginMulticast(IPAddress interfaceAddr, IPAddress multicast, uint16_t port)
 {
-    if (_ctx)
-    {
+    if (_ctx) {
         _ctx->unref();
         _ctx = 0;
     }
 
-
     ip_addr_t ifaddr;
     ifaddr.addr = (uint32_t) interfaceAddr;
     ip_addr_t multicast_addr;
@@ -95,6 +93,7 @@ uint8_t WiFiUDP::beginMulticast(IPAddress interfaceAddr, IPAddress multicast, ui
     }
 
     _ctx = new UdpContext;
+    
     if (!_ctx->listen(*IP_ADDR_ANY, port)) {
         return 0;
     }
@@ -134,9 +133,8 @@ int WiFiUDP::beginPacket(IPAddress ip, uint16_t port)
     ip_addr_t addr;
     addr.addr = ip;
 
-    if (_ctx)
-        _ctx->unref();
-    _ctx = new UdpContext;
+    if (!_ctx)
+        _ctx = new UdpContext;
     return (_ctx->connect(addr, port)) ? 1 : 0;
 }
 
@@ -227,3 +225,10 @@ uint16_t WiFiUDP::remotePort()
     return _ctx->getRemotePort();
 }
 
+uint16_t WiFiUDP::localPort()
+{
+    if (!_ctx)
+        return 0;
+
+    return _ctx->getLocalPort();
+}

hardware/esp8266com/esp8266/libraries/ESP8266WiFi/src/WiFiUdp.h

@@ -86,6 +86,8 @@ class WiFiUDP : public UDP {
   virtual IPAddress remoteIP();
   // Return the port of the host who sent the current incoming packet
   virtual uint16_t remotePort();
+  // Return the local port for outgoing packets
+  uint16_t localPort();
 
 };
 

hardware/esp8266com/esp8266/libraries/ESP8266WiFi/src/include/UdpContext.h

@@ -120,6 +120,14 @@ class UdpContext
         return ntohs(udphdr->src);
     }
 
+    uint16_t getLocalPort()
+    {
+        if (!_pcb)
+            return 0;
+
+        return _pcb->local_port;
+    }
+
     bool next()
     {
         if (!_rx_buf)