Sending long int over serial (byte array conversion)

[riccardo-m]

Hi,

I've been trying to send a long integer from an Arduino to my C++ program via the serial port. There seems to be many examples of this when i search, but I've not had much luck except for one example. This example seems to work, but not when I send a negative integer. I'm not sure if it is my arduino code or my c++ code, or both that needs changing. Does anyone know how I could adjust this to work with negative numbers too?

// Ardunino Code - Sends 4 bytes to C++ program on PC
void IntegerToBytes(long val, byte b[4]) {
  b[0] = (byte )((val >> 24) & 0xff);
  b[1] = (byte )((val >> 16) & 0xff);
  b[2] = (byte )((val >> 8) & 0xff);
  b[3] = (byte )(val & 0xff);
}

long n = 23580; 
byte b[4];
IntegerToBytes(n, b);
for (int i=0; i<4; ++i) {
    Serial.write((int )b[i]);
}

-

// C++ code to convert the captured bytes back into a long integer
    long int int32 = bytesIn[3] | ( (int)bytesIn[2] << 8 ) | ( (int)bytesIn[1] << 16 ) | ( (int)bytesIn[0] << 24 );

If I send 23580, int32 becomes 23580
If I send -23580, int32 becomes -28

[Ancient Dragon]

instead of all that shifting I just store it in a char array then xmit the array

inx x = -123;
unsigned char buf[sizeof(int)];
memcpy(buf,&x,sizeof(int);
// now just xmit buf
 Serial.write(buf,sizeof(buf));


 // C++ code to convert the captured bytes back into a long integer
 int x;
 memcpy(&x,buf,sizeof(int));

 // or this works too
 x = *(int *)buf;

Not considered in the above code is endianess
Binary data is not directly transferrable between MS-Windows and *nix because the byte order is different.

[deceptikon]

instead of all that shifting I just store it in a char array then xmit the array

This introduces a portability issue that doesn't exist with shifting (ie. byte order), though whether that matters to the OP hasn't been established.

[Ancient Dragon]

There are ways to handle that too.

[riccardo-m]

Thanks guys. That sorted it. I tried keeping the bit shifting method on the C++ part (so it would be more portable, allthough it doesn't really matter for my application), but it seemed to half work. I tried the memcpy though and that seems to work just fine.

// Arduino Code
long int x = -123;
unsigned char buf[sizeof(long int)];
memcpy(buf,&x,sizeof(long int));
Serial.write(buf,sizeof(buf));


// C++ Code
memcpy(&intVal,buf,sizeof(long int)); //This works fine

intVal = buf[0] | ( (int)buf[1] << 8 ) | ( (int)buf[2] << 16 ) | ( (int)buf[3] << 24 );
// This gives the following results...
// send -123 intVal = -123
// send 3687 intVal = 3687
// send -68732 intVal = -124  <---- ?