Serial communication: Sending data from C++ to Arduino - c++

Question

i'm trying to send data from a C++ program over serial communication to an Arduino. I formed a struct for sending the data as an object:
typedef struct
{
double width;
double height;
bool passBoard;
} MachineParameters;
I'm using this Serial library: http://wjwwood.github.com/serial/ for sending the data like this:
MachineParameters mp;
mp.width = 100;
mp.height = 200;
mp.passBoard = true;
ser.write((const uint8_t *)&mp, sizeof(mp));
The library makes it possible to send the data as uint8_t, std::vector or std::string.
The Arduino does receive data, but i don't know how to parse the data into the struct. I'm using the same struct as in the cpp code.
// In Arduio
MachineParameters mp;
int byte_size = 24;
loop() {
if(Serial.available() >= 24) {
Serial.readBytes((char*) &mp , 24);
}
}
// Goal: Read received mp data just like
// mp.width or mp.height
After hours of trying, i still cannot figure it out, how to send this struct to the arduino successfully. Is there another way of sending this data to the arduino? It worked sending the data as string, but that did not seem right.
I am pretty new to programming with C++, so please excuse any obvious questions...
Thank you for helping!
UPDATE: Working solution below
After a view more tries and thanks to your tips, i figured it out. Here is the code, which worked for me. I found out that my problem was the wrong byte size, used for parsing the buffer. The size of the struct in C++ is 12, whereas on the arduino it's 9. Using the original size (12) for parsing the buffer on the Arduino, the struct was parsed correctly.
/* --- C++ CODE --- */
typedef struct
{
double width;
double height;
bool passBoard;
} MachineParameters;
// sizeof(MachineParameters) returns 12.
MachineParameters mp;
mp.width = 11.1;
mp.passBoard = false;
mp.height = 22.2;
ser.write((uint8_t *)&mp, sizeof(mp));
/* --- END OF C++ --- */
/* --- Arduino Code --- */
#define BYTE_SIZE 12
char messageBuffer[BYTE_SIZE];
typedef struct
{
double width;
double height;
bool passBoard;
} MachineParameters;
MachineParameters mp;
void setup() {
Serial.begin(9600);
}
void loop() {
if (Serial.available() >= BYTE_SIZE) {
Serial.readBytes(messageBuffer , BYTE_SIZE);
memcpy(&mp, &messageBuffer, BYTE_SIZE);
// mp.width returns 11.1
// Success :)
}
}
/* --- END OF ARDUINO --- */

Answer 1

on the arduino you receive the data in a buffer as raw bytes
now you have to parse this, you can do this using memcpy to copy the data in the struct
however for doing this the data has to be aligned meaning that you have to know exactly where it begins. so you should send a synchronization byte ( start / stop bytes ) to be able to fix where the data begins
then you can use the code in parsing buffer data into struct :
struct abc {
char a;
char b;
char c;
char d[2];
};
int main() {
char arr[5] = { 'a', 'b', 'c', 'd', 'e' };
struct abc sa;
memcpy(&sa, arr, 5);
return 0;
}
here arr is incoming buffer, and with memcpy all the contents are copied appropriately.
ser.write((const uint8_t *)&mp, sizeof(mp)); is correct
you can create an identical structure mp_arduino on the arduino and copy the content of the receive buffer into this structure with
memcpy ( &mp_arduino, receive_buffer+n, sizeof(mp_arduino) ); where n indicates the position / byte in receive_buffer where your data begins and receive_buffer is defined as uint8_t receive_buffer[] ( or uint8_t* receive_buffer is the same... )
( http://www.cplusplus.com/reference/cstring/memcpy/ )
this is how a struct is stored in memory : How is a struct stored in memory?
on both systems a float should be 32 bit https://www.arduino.cc/reference/en/language/variables/data-types/float/