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Re: Compiler warning about comparison always false

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On 07.03.2012 18:25, Nachy wrote:
> Hi, all.
> 
> I got an interesting warning message from my compiler, and before I
> complain about a bug I want to make sure what's going on.
> 
> I have a method  disp_char, meant to append a character to a string,
> or a period if the character is a control character. The string better
> be the right size (it is, that's not the issue).
> 
> int disp_char(int c, char *s)
> {
> 	char ch = c & 0xFF;						// make sure it's a char
> 	if (ch < 0x20 || ch == 0x7F || ch == 0xFF)		// special character?
> 		*s = '.';
> 	else
> 		*s = ch;
> 	*++s = '\0';
> 
> 	return 1;
> }
> 
> On one system I compiled with, gcc gave me a warning:
> $ gcc -o ../dump dump.c
> dump.c: In function `disp_char':
> dump.c:140: warning: comparison is always false due to limited range
> of data type
> 

If you want ch to be unsigned, why don't you tell the compiler? But why
go such lengths at all? c can have those values, too, and more. If c is
the result from a call to fgetc, you should check for an error or
end-of-file by comparing it to EOF.

The result of fgetc() is defined to be either a character or EOF. Also
you can make this function 100% portable by including <ctype.h> and
replacing the condition of the if with !isprint(c).

> I separated the comparisons, and gcc seems to be complaining about the
> last one (ch == 0xFF).
> 
> Am I doing something wrong, or is gcc being paranoid?
> 

char may be signed or unsigned. Usually an integer type without special
qualifier is signed, but char is special. Basically, int is defined to
be the same as signed int, but char is defined to be the same as signed
char _or_ unsigned char. Programmers usually cope by treating char,
signed char and unsigned char three distinct types and try not to make
use of arithmetic on char other than what is defined to work. For
instance, the C standard defines the digit characters to be in order,
but not the letters. That means

r = 0;
while (isdigit(*s))
    r = 10 * r + *s++ - '0';

is a portable way to parse a decimal number, but

r = 0;
while (isxdigit(*s))
    if (isdigit(*s)
        r = 16 * r + *s++ - '0';
    else if (isupper(*s))
        r = 16 * r + *s++ - 'A' + 10;
    else
        r = 16 * r + *s++ - 'a' + 10;

is not a portable way to parse a hexadecimal number. If you want to make
it 100% portable, you'd have to do something like:

char digits[] = "0123456789ABCDEF";
unsigned int r = 0;
char c, *p;
for (c = *s; c; c = *++s)
{
    if (islower(c)) c = toupper(c);
    p = strchr(digits, c);
    if (p) r = 16 * r + (p - digits);
    else break;
}

> BTW, on another system, gcc did not complain at all, even with -Wall.
> The program seems to work fine, although I can't seem to generate a
> test case with an actual file.
> 

Try compiling with --signed-char on those systems.

> Thanks.
> 
> -- Nachy

Please insert a newline after the dash-dash-space, then it gets treated
as signature and is cut off automatically when answering.

Ciao,
Markus

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Thread

Compiler warning about comparison always false Nachy <mazwolfe@gmail.com> - 2012-03-07 09:25 -0800
  Re: Compiler warning about comparison always false bert <bert.hutchings@btinternet.com> - 2012-03-07 09:32 -0800
    Re: Compiler warning about comparison always false Nachy <mazwolfe@gmail.com> - 2012-03-07 09:42 -0800
      Re: Compiler warning about comparison always false Philip Lantz <prl@canterey.us> - 2012-03-07 10:17 -0800
        Re: Compiler warning about comparison always false Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-13 15:30 -0700
          Re: Compiler warning about comparison always false Philip Lantz <prl@canterey.us> - 2012-03-14 00:55 -0700
      Re: Compiler warning about comparison always false James Kuyper <jameskuyper@verizon.net> - 2012-03-07 14:06 -0500
        Re: Compiler warning about comparison always false Fred <fred.l.kleinschmidt@gmail.com> - 2012-03-07 11:11 -0800
  Re: Compiler warning about comparison always false "io_x" <a@b.c.invalid> - 2012-03-07 18:51 +0100
  Re: Compiler warning about comparison always false falk@rahul.net (Edward A. Falk) - 2012-03-07 20:29 +0000
    Re: Compiler warning about comparison always false Keith Thompson <kst-u@mib.org> - 2012-03-07 14:36 -0800
  Re: Compiler warning about comparison always false Markus Wichmann <nullplan@gmx.net> - 2012-03-07 21:26 +0100
  Re: Compiler warning about comparison always false Kaz Kylheku <kaz@kylheku.com> - 2012-03-14 17:49 +0000

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