magnus%thep.lu.se@Urd.lth.se (Magnus Olsson) (02/26/91)
In Turbo C++ on an IBM AT, the following code fragment
{
unsigned char u,v;
signed char s;
u = 255;
s = (signed char) u;
v = (unsigned char) s;
}
will assign the value -1 to s and 255 to v. This isn't surprising,
since these values have the same bit pattern (11111111) on a 2's
complement machine.
The question is - is this construct portable?
I have a suspicion that it will give some other
result on a machine that doesn't use 2's complement notation -
on a 1's complement machine, for example, the bit pattern
11111111 interpreted as a signed byte is "-0", not -1. Or
is the way the conversion should be done specified in the
C language (so it's always done the same way)?
Perhaps it would be better to write
s = (u > 127) ? (int) u - 256 : u;
v = (s < 0) ? s + 256 : s;
(provided, of course, that one really wants the conversion to be
done as under Turbo C++)?
Magnus Olsson | \e+ /_
Dept. of Theoretical Physics | \ Z / q
University of Lund, Sweden | >----<
Internet: magnus@thep.lu.se | / \===== g
Bitnet: THEPMO@SELDC52 | /e- \q