Re: [Sdcc-user] overflow in supposed to be unsigned long

[Erik P. <epe...@iv...>]

On Wed, 4 May 2011, st...@sk... wrote:

> Hi, I have to transmit an unsigned long as four bytes, so I split them up
> and join them again, but with the folowing example code, I get overflow at
> long_counter>32767 - can any of you give me a clue how to do it right?

>	// split long_counter to four unsigned char's
>	for (i = 0; i < 4; i++) {
>		char_counter[i] = (long_counter & (0xff << (i * 8))) >> (i * 8);
> 	}
>
>	// convert them to unsigned int again
>	long_counter = char_counter[0] | (char_counter[1] << 8) |
>	              (char_counter[2] << 16) | (char_counter[3] << 24);
>

Your code for splitting the long into chars should work, but SDCC can 
optimize this much better:

   char_counter[0] = long_counter;
   char_counter[1] = long_counter >> 8;
   char_counter[2] = long_counter >> 16;
   char_counter[3] = long_counter >> 24;

The problem you are having putting them back together is due to the C 
integer promotion rules. Although you store the final result into a 
variable of type long, all of the math leading up to that result will be 
calculated with an int sized intermediary. Typecasting is needed to 
explicitly promote the values to a larger type so that the intermediary 
compuation does not overflow:

   long_counter = (char_counter[0] | (char_counter[1] << 8)) |
     ((unsigned long)(char_counter[2] | (char_counter[3] << 8)) << 16);

This combines pairs of bytes into two int sized values, then promotes one 
of them to a long sized value (the other will then also be promoted to the 
same size automatically) before combining them into a single long.

   Erik