How to determine whether a variable is unsigned or not in ANSI C

Question

I am learning "Expert C Programming" by Peter Van Der Linden. In chapter A.6, the writter described how to determine whether a variable is unsigned or not in K&R C.The macro is below:

#define ISUNSIGNED(a) (a>=0 && ~a>=0)

The book is very old, it was first published in 1994! And I have not learned K&R C before. The question is that how to determine whether a variable is unsigned or not in ANSI C.

I have tried to solve the problem like this. Since "0" is int in ANSI C, and any other number except float, double and long double will be converted to int or unsigned int by Integer Upgrade when compare with 0. So I want to find an edge between unsigned and signed number. When I compare (the edge type)0 with a, the type of a will not be changed. The macro also the model is below:

#define ISUNSIGNED(a) (a>=(the edge type)0 && ~a>=(the edge type)0)

I can not find the edge type, is there anybody can help me solve the problem? I have changed "number" to "variable" for more accurate expression.

Answer 1

For anyone coming to this question but not restricted to C89 (technically C11 is also ANSI C because ANSI has ratified it):

#include <stdio.h>
#include <limits.h>

#define ISSIGNED(x) _Generic((x), \
    unsigned char: 0,       \
    unsigned short: 0,      \
    unsigned int: 0,        \
    unsigned long: 0,       \
    unsigned long long: 0,  \
    signed char: 1,         \
    signed short: 1,        \
    signed int: 1,          \
    signed long: 1,         \
    signed long long: 1,    \
    char: (CHAR_MIN < 0)    \
)

int main()
{
    char x;
    printf("%d\n", ISSIGNED(x));
}

Answer 2

#define ISUNSIGNED(a) (a>=0 && ((a=~a)>=0 ? (a=~a, 1) : (a=~a, 0)))