Question

How do I write a macro CHECK(a, b) that generates compilation error when the two pointers a & b have different base type.

CHECK((int*)0, (char*)0) -> compilation error
CHECK((int*)0, (int*)0) -> works

I’m looking for some C89 code, but C99 + gcc extensions will also do.

Answer 1

EDIT now works for any type, not just pointers

Something more or less lifted from the linux kernel, using the GCC extension typeof().

This generates a warning at compile time, it also works for integer pointer types

#define CHECK(a, b) do { \
    typeof(a) _a; \
    typeof(b) _b; \
    (void) (&_a == &_b); \
    } while (0)

int main(int argc, char **argv)
{
    int *foo;
    int *bar;
    char *baz;

    CHECK(foo, bar);
    CHECK(bar, baz);
    return 0;
}