Checks for the presence of a UDP packet, and reports the size. parsePacket() must be called before reading the buffer with UDP.read().

UDP.parsePacket();

None

int: the size of a received UDP packet

#include <SPI.h>         // needed for Arduino versions later than 0018
#include <Ethernet.h>
#include <EthernetUdp.h>         // UDP library from: bjoern@cs.stanford.edu 12/30/2008
 
 
// Enter a MAC address and IP address for your controller below.
// The IP address will be dependent on your local network:
byte mac[] = {  
  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
IPAddress ip(192, 168, 1, 177);
 
unsigned int localPort = 8888;      // local port to listen on
 
// An EthernetUDP instance to let us send and receive packets over UDP
EthernetUDP Udp;
 
void setup() {
  // start the Ethernet and UDP:
  Ethernet.begin(mac,ip);
  Udp.begin(localPort);
 
  Serial.begin(9600);
}
 
void loop() {
  // if there's data available, read a packet
  int packetSize = Udp.parsePacket();
  if(packetSize)
  {
    Serial.print("Received packet of size ");
    Serial.println(packetSize);
  }
  delay(10);
}

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