C equivalent of Python None type

Python is dynamically typed so you can bind a variable name to any type, such as string, numeric, or NoneType. As C is statically typed, variables are locked to a specific type, but there's nothing stopping you from creating a type able to be any type.

For example, a union with a tag field to indicate the type, such as in the following complete program. First, the tag and union types which allow you to store and select any of the types:

enum uberType { ETYP_NONE, ETYP_INT, ETYP_DOUBLE, ETYP_CHARPTR };
typedef struct {
    enum uberType type;
    union { int valInt; double valDouble; char *valCharPtr; };
} tUberType;

Then some helper functions to set the uber-type to a specific typed value:

void makeNone(tUberType *ut) {
    ut->type = ETYP_NONE;
}

void makeInt(tUberType *ut, int val) {
    ut->type = ETYP_INT;
    ut->valInt = val;
}

void makeDouble(tUberType *ut, double val) {
    ut->type = ETYP_DOUBLE;
    ut->valDouble = val;
    }

void makeCharPtr(tUberType *ut, char *val) {
    ut->type = ETYP_CHARPTR;
    ut->valCharPtr = val;
}

And, finally, a test harness, including an output function:

#include <stdio.h>

void evalUber(tUberType *ut, char *post) {
    switch (ut->type) {
    case ETYP_NONE:
        printf("none:%s", post);
        break;
    case ETYP_INT:
        printf("int:%d%s", ut->valInt, post);
        break;
    case ETYP_DOUBLE:
        printf("double:%f%s", ut->valDouble, post);
        break;
    case ETYP_CHARPTR:
        printf("charptr:%s%s", ut->valCharPtr, post);
        break;
    default:
        printf("?%s", post);
        break;
    }
}

int main(void) {
    tUberType x;
    makeNone(&x); evalUber(&x, "\n");
    makeInt(&x, 42); evalUber(&x, "\n");
    makeDouble(&x, 3.14159); evalUber(&x, "\n");

    return 0;
}

The output of the test harness main is, as expected:

none:
int:42
double:3.141590