From: david.br...@hesbynett.no
[...char vs. signed cahr vs. unsigned char...]
>
> Don't look it up - if you learn the compiler's default behaviour, you
> risk writing incorrect code. If it is in any way relevant whether a
> char is signed or not, write "signed char" or "unsigned char" explicitly
> in the code. The only reason one would ever use a plain "char" is to
> hole a true character - a letter - such as in strings. Since it is (in
> general) meaningless to do arithmetic on characters, it does not matter
> if the compiler considers them to be signed or unsigned.
All true, but you need to be careful all the same. Consider the case I ran
into about 20 years ago, a custom terminal with "Function" keys (e.g., F1, F2,
etc) that returned values outside the standard ASCII range (0xF1, 0xF2, etc.).
Consider the following code, stubbed out for illustration purposes:
#define EXIT_KEY 0xF1
extern void process_key(char key);
char get_key(void) {return EXIT_KEY}
void handle_input(void)
{
char key;
do
{
key = get_key();
process_key(key);
} while (key != EXIT_KEY);
}
In this code, the handle_input function falls into an infinite loop if plain
char is signed. This is because key gets promoted (to signed int) before the
comparison with EXIT_KEY (which is already signed int). If plain char is
signed, and key is 0xF1, the sign is extended for key, but not EXIT_KEY.
There are numerous solutions, but I post it here as a warning to the wise.
Sometimes math is performed on character types implicitly.
Regards,
-=Dave
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