Sending a struct from Arduino to Raspberry pi via Ethernet

Projects Programming
1

Hi,

I've been trying to send a simple packet from an Arduino to a Raspberry pi. I got it working with a struct containing 2 chars, but when it's a char and an int/float I only get the first part (See the output at the bottom).

The client sends the string 'temp' to the server which creates a packet with a char value of 'A', and a float value of '3'. Then I memcpy the packet in to a buffer, and send bytes equal to the size of the packet. On the client side I memcpy the buffer back to a packet and print it to the screen.

Server side - Arduino

#include <Ethernet.h> //Load Ethernet Library
#include <EthernetUdp.h> //Load UDP Library
#include <SPI.h> //Load the SPI Library

byte mac[] = { 0x00, 0xAC, 0xBF, 0xCC, 0x02, 0x08 }; //Assign a mac address
IPAddress ip(192, 168, 1, 177); //Assign my IP adress
unsigned int localPort = 5000; //Assign a Port to talk over
char packetBuffer[UDP_TX_PACKET_MAX_SIZE];
String datReq; //String for our data
int packetSize; //Size of Packet
EthernetUDP Udp; //Define UDP Object

//TMP36 Pin Variables
int sensorPin = 0;

struct Packet {
    char val;
    float temp;
};

void setup() {
    Serial.begin(9600); //Turn on Serial Port
    Ethernet.begin(mac, ip); //Initialize Ethernet
    Udp.begin(localPort); //Initialize Udp
    delay(1500); //delay
}

void loop() {
    packetSize = Udp.parsePacket(); //Read the packetSize

    if (packetSize>0) { //Check to see if a request is present
        Udp.read(packetBuffer, UDP_TX_PACKET_MAX_SIZE); //Reading the data request on the Udp
        String datReq(packetBuffer); //Convert packetBuffer array to string datReq
                                     //Serial.println(datReq);

        if (datReq == "temp") { //See if Red was requested
            Packet pkt;

            pkt.val = 'A';
            pkt.temp = 3;


            Udp.beginPacket(Udp.remoteIP(), Udp.remotePort());  //Initialize Packet send

            char txBuffer[128] = { 0 };
            memcpy(txBuffer, &pkt, sizeof(Packet));

            Udp.write(txBuffer, sizeof(Packet));
            Udp.endPacket(); //Packet has been sent
        }

        memset(packetBuffer, 0, UDP_TX_PACKET_MAX_SIZE);
    }
}

Client side - Raspberry pi

//See Description.txt

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <iostream>
using std::cout;
using std::endl;

struct Packet{
	char val;
	float temp;
};

int main(void)
{
    int fd;
    if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        cout << "socket failed" << endl;
        return 1;
    }

    struct sockaddr_in serveraddr;
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_port = htons(5000);
    serveraddr.sin_addr.s_addr = inet_addr("192.168.0.25");

	for(int i = 0; i < 5; i++){
		Packet pkt;
		pkt.temp = 0;
		sendto(fd, "temp", 4, 0, (struct sockaddr *)&serveraddr, sizeof(serveraddr));
		
		char rxBuffer[128] = {0};
		
		socklen_t addr_len = sizeof(serveraddr);
		recvfrom(fd, rxBuffer, sizeof(struct Packet), 0, (sockaddr *)&serveraddr, &addr_len);		
		
		memcpy(&pkt, rxBuffer, sizeof(struct Packet));
		cout << pkt.val << " : " << pkt.temp << endl;
	}

    close(fd);
}

Output:
A : 9.24857e-44
A : 9.24857e-44
A : 9.24857e-44
A : 9.24857e-44
A : 9.24857e-44

Can someone point out the problem here please?

2 
        String datReq(packetBuffer); //Convert packetBuffer array to string datReq

There is NO reason to wrap the char array in a String just so you can use == instead of strcmp().

floats on the Pi are not the same size as floats on the Arduino.

The structs on the two devices are not the same size.

3

Ahh the size of the struct in the Arduino is 5 bytes, the struct in the Pi is 8.

Thank you