'comparison is always true due to limited range of data type' warning in C?

Question

I have the following code

//Point.h
#define WIDTH 8
#define HEIGHT 8

typedef struct Point
{
  char x;
  char y;
} Point;

//Board.c
#include <stdbool.h>

// Some other functions that we don't care about... 

bool inBounds(Point * p)
{
  return p->x >= 0
    && p->x <= WIDTH
    && p->y >= 0
    && p->y <= HEIGHT;
}

When I compile this (ppu-gcc 4.1.1), I get the following warning

warning: comparison is always true due to limited range of data type

even though the range of char is -127 to 127 and WIDTH is 8, which is well inside the range of a char. I've already tried an explicit cast of WIDTH to a char, but still got the error.

Answer 1

Are you sure that char is signed? Try declaring the fields explictly as signed char and see what you get.

Answer 2

I guess x >= 0 causes the warning because char might be implemented as unsigned char.

Answer 3

The char type may be signed or unsigned. It depends on your compiler vendor's choice. There might even be a compiler option available. Evidently, char is unsigned for you, so it's always greater than or equal to zero, and thus the compiler warns you.

You're using char here to represent "a numeric type that takes up minimal memory." In that case, I recommend explicitly using signed char or unsigned char. (Each is distinct from plain char, despite char having to be either signed or unsigned.) Reserve char for when you're holding character data. For numeric data, use one of the other two types.

Answer 4

Hummm... isn't your char unsigned by default? In that case the range would be 0-255, which means your >=0 comparison would be always true

Answer 5

The C and C++ standards allows the character type char to be signed or unsigned, depending on the platform and compiler. Most systems, including x86 GNU/Linux and Microsoft Windows, use signed char, but those based on PowerPC and ARM processors typically use unsigned char.(29) This can lead to unexpected results when porting programs between platforms which have different defaults for the type of char.

Answer 6

Try this:

typedef struct Point
{
  signed char x;
  signed char y;
} Point;