Bitwise operations, please help me not go mental!

Forum 2005-2010 (read only) Software Syntax & Programs
1

Dear All,

I am struggling with some basic bitwise operations, namely converting a long int to bytes, and back again.
This topic is quite well covered on a lot of places, inlcuding this forum.
Yet, I can't figure out why its not working.

I completly forget about why I need this, just a very basic example and even that won't work as expected. So if you would be so kind to help me point out the error of my ways I would be very thankfull indeed!

void loop () {
  long n = 1234567890;
  byte buf[4];
  buf[0] = (byte) n;
  buf[1] = (byte) n >> 8;
  buf[2] = (byte) n >> 16;
  buf[3] = (byte) n >> 24;
  long value = (unsigned long)(buf[3] << 24) | (buf[2] << 16) | (buf[1] << 8) | buf[0];
  
  Serial.print("input value: ");
  Serial.println(n, BIN);
     
  Serial.print("returned value: ");
  Serial.println(value, BIN);
  
  Serial.print("returned INT: ");
  Serial.println(value);
  
  delay(1000);
}

This returns an int of 210, which is obviously not the same value as I started with

buf[] contains what I expect though. So What I need at this point is to "concatenate" the 4 bytes in buf[] in such a way that it returns the original long int.

Any help would be very much appreciated!

2

Try adding an L to the end of your long constant.

3

Makes no difference in above example.

4

You may want to look at how you are taking 'n' apart.
Your operators are fine, but precedence is a problem.

(sorry, posted from my phone before - couldn't see all the code)

buf[] contains what I expect though

Really?

5

Precedence errors in disassembly and cast error in reassembly of the 32 bit value. Corrected sketch:

void setup() {
  Serial.begin(9600);
  
  long n = 1234567890;
  byte buf[4];

  buf[0] = (byte) n;
  buf[1] = (byte) (n >> 8);
  buf[2] = (byte) (n >> 16);
  buf[3] = (byte) (n >> 24);

  long value = ((unsigned long)buf[3]) << 24 | 
               ((unsigned long)buf[2]) << 16 |
               ((unsigned long)buf[1])<< 8
               | buf[0];

  Serial.print("input value: ");
  Serial.println(n, BIN);

  Serial.print("returned value: ");
  Serial.println(value, BIN);

  Serial.print("returned INT: ");
  Serial.println(value);

  exit(0);
}

void loop () {
}
6

Ok, that works.

But looking at the above version and my version, the only difference seems to be the cast to an unsigned long on each byte, correct?

7

No, not at all correct.
The long cast is on the reassembly, but the disassembly is also changed, which is why I queried your assertion the contents of 'buf' were correct.

Casting a 32 bit value to an eight bit value, and then dividing by 256 isn't going to work.