How to check if a variable is of "const" qualifier type in C?

Question

Sample code to check

#include<stdio.h>

int main(void)
{
    const int i = 1;
    printf("Variable i is %s\n",
           __builtin_constant_p(i) ? "a const variable" : "not a const variable");
    return 0;
}

Output :

Variable i is not a const variable

Is __builtin_constant_p() not the right API to determine whether a variable is of type const or not?

Answer 1

You can use Generic selection (since C11):

#include <stdio.h> 

#define __is_constant_int(X) _Generic((&X), \
        const int *: "a const int", \
        int *:       "a non-const int")

int main(void)
{
    const int i = 1;
    printf("Variable i is %s\n", __is_constant_int(i));
    return 0;
}

Answer 2

RTFineManual:

You can use the built-in function __builtin_constant_p to determine if a value is known to be constant at compile time ..."

(emphasis mine) Note this is a gcc compiler extension, thus not standard compliant.

C does not have symbolic constants other than enum-constants. const qualified objects are still variables. Thus the test fails. See the manual for a typical application.

---

Notes:

Assuming the code shown is just an example, your question looks like an XY-problem. C is statically typed, thus at the point you use such a construct the full type is well known. The only way to have something function-like would be a macro. But hiding different behaviour in a macro will cause confusion by the reader of the code. She has to remember this difference for every invokation.

Instead use two functions with the different behaviour and name them accordingly. As the caller know the type he can use the correct function and any reader of the code (including you some weeks later!) will instantly know about the difference.