On Wed, 4 May 2011, [email protected] 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
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