C variable and constant value comparison not matching

Question

If I have:

signed char * p;

and I do a comparison:

if ( *p == 0xFF )
   break;

it will never catch 0XFF, but if I replace it with -1 it will:

if ( *p == (signed char)0xFF )
   break;

How can this happen? Is it something with the sign flag? I though that 0xFF == -1 == 255.

Answer 1

The value 0xFF is a signed int value. C will promote the *p to an int when doing the comparison, so the first if statement is equivalent to:

if( -1 == 255 ) break;

which is of course false. By using (signed char)0xFF the statement is equivalent to:

if( -1 == -1 ) break;

which works as you expect. The key point here is that the comparison is done with int types instead of signed char types.