offsetof-test.c

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

typedef struct _hoge {
    int a;
    char c;
    float f;
} Hoge;

int main(int argc, char* argv[])
{
    printf(
        "%d %d\n",
        offsetof(Hoge, f),
        &(((Hoge*)NULL)->f)
    );
    return 0;
}

output.txt

$ gcc offsetof-test.c
$ ./a.out
8 8