Wrong received message using UDP server- Solved

Projects Networking, Protocols, and Devices
1

Dear All,

I'm trying to communicate between two systems using UDP server, the 1st system send a message in this form as example 0024008c0a00000050000000000000000000000000000000000000006400000090010000
the total length is 36 bytes, where byte 0 is 0 always, byte 1 is 36 always, byte 2 is 0 always, byte 4-7 is variable0 the type is INT32, byte 8-11 is variable1 the type is INT32,...., until 32-35 which is the last variable7.
in the message above the value for var1 is 10 , var2 = 80, var6 = 100 and var7 is 400.

but when i use this code to read the UDP packet i got different message .

The code

#include <ESP8266WiFi.h>
#include <WiFiUdp.h>
   

#define WIFI_SSID "****"
#define WIFI_PASS ""
#define UDP_PORT 9180

WiFiUDP UDP;
char packet[255];
char reply[] = "Packet received!";
signed long INT0;
signed long INT1;
signed long INT7;
signed long INT6;

void setup() {

  Serial.begin(115200);
  Serial.println();
   

  WiFi.begin(WIFI_SSID , WIFI_PASS);
   

  Serial.print("Connecting to ");
  Serial.print(WIFI_SSID );
  Serial.print(WIFI_PASS);
  // Loop continuously while WiFi is not connected
  while (WiFi.status() != WL_CONNECTED)
  {
    delay(100);
    Serial.print(".");
  }
   

  Serial.println();
  Serial.print("Connected! IP address: ");
  Serial.println(WiFi.localIP());

  // Begin listening to UDP port
  UDP.begin(UDP_PORT);
  Serial.print("Listening on UDP port ");
  Serial.println(UDP_PORT);
   
}
   
void loop() {

  int packetSize = UDP.parsePacket();
  if (packetSize) {
    Serial.print("Received packet! Size: ");
    Serial.println(packetSize); 
    int len = UDP.read(packet, 255);
   if (len > 0)
    {
      packet[len] = '\0';
    }

for (int i = 0; i < packetSize ; i = i + 1) {
 // Serial.print(i);

  Serial.print(packet[i],HEX);

Serial.println();
    // Send return packet
    UDP.beginPacket(UDP.remoteIP(), UDP.remotePort());
    UDP.write(reply);
    UDP.endPacket();
 
  }

I'm getting this message

Received packet! Size: 36
02408CA0005000000000000000000006400090100

I really need some help since i tried so many times to know what causing the issue.

2

Values in the printed message matches what is indicated in the header.
Your message is printed shorter because you are not printing leading zeros.

Edit your print loop to:

1 Like
3

In general, printing hex values on one line without separators is a bad idea.
For example, the line 00AA13- what is it?
0 0A A1 3 (0x00 0x0A 0xA1 0x03)
or
0 0 AA 13 (0x00 0x00 0xAA 0x13)
?

4

Thank you so much, i didn't think about it before, you made my day sir.
Can you give me a clue about trimming it to get bytes 4-7 only as one variable ?

Thank you

5

let's assume that bytes 4-7 is uint32_t variable:

uint32_t x;                                  // variable for storing value of 4-7 bytes
memcpy(&x,(uint8_t*)packet +4, sizeof(x));  //copy 4 bytes starting from packet[4] to variable x
1 Like
6

Man, i don't know what to say, thank you so much for your help.

8

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