Should this char be unsigned?

Question

I found some confusing code during code review and am a bit puzzled. Doing some research I found this situation. I wrote this sample of code to highlight the problem

char d = '©';// this is -87,the copyright symbol , (actually its 169 unsigned)
if(ispunct(d)) // will assert. 
{        
}

so, the programmer who was bug fixing, did the following:

char d = '©';// this is -87,the copyright symbol , (actually its 169 unsigned)
if(ispunct((unsigned char)d)) // will not assert, because it will be 169.
{        
}

My question is whether it is OK to make the char unsigned ? Ideally, I wouldn't use char but use a Unicode char to avoid this kinds of problem, but the software is very old and wont be reengineered any time soon.

I am using Visual Studio 2008. ispunct() can be found in ctype.h.

Answer 1

The cast is correct in this case. From man ispunct:

     The ispunct() function tests for any printing character except for space
     (` ') or a character for which isalnum(3) is true.  The value of the
     argument must be representable as an unsigned char or the value of EOF.

Answer 2

It's perfectly legitimate to do the cast. I believe in C variables are signed by default but the cast makes it useable.

Answer 3

If you want to use ispunct, then there's no way around it.

_ASSERTE((unsigned)(c + 1) <= 256);

That's what caused the assertion to fail and hence the cast is appropriate.