Own offsetof implementation produces warning

Question

I wrote a small piece of code to understand how the offsetof macro works in the background. Here is the code:

#include <stdio.h>   
#include <stdlib.h>  

int main(void)
{
    /* Getting the offset of a variable inside a struct */
    typedef struct {
        int a;
        char b[23];
        float c;
    } MyStructType;

    unsigned offset = (unsigned)(&((MyStructType * )NULL)->c);

    printf("offset = %u\n", offset);

    return 0;
}

However, if I run it I get a warning message:

WARNING: cast from pointer to integer of different size [-Wpointer-to-int-cast]

However, if I look at the original offsetof macro in c, the code looks like this:

#include <stdio.h>   
#include <stdlib.h> 
#include <stddef.h> 

int main(void)
{
    /* Getting the offset of a variable inside a struct */
    typedef struct {
        int a;
        char b[23];
        float c;
    } MyStructType;

    unsigned offset = offsetof(MyStructType, c);

    printf("offset = %u\n", offset);

    return 0;
}

So why do I get the warning as I cast to unsigned ? It appears to be the type for the offsetof macro. This is puzzling me.

Answer 1

As mch commented, unsigned is not the right type; it's 32-bit on pretty much all real-world systems. The offsetof macro is supposed to produce a result of type size_t , which is what you "should" be casting to here. I think you're confused by the code you found storing the result into an object of type unsigned ; that's okay as long as they're sure the value is small, but it doesn't mean the type of the expression offsetof(MyStructType, c); was unsigned . C allows you to silently assign a larger integer type into a smaller one.

However, no matter what you do, this is not a valid implementation of offsetof and has undefined behavior (via applying -> to an invalid pointer). The way you get a working offsetof without UB is #include <stddef.h> .