struct_padding.c

#include <stdio.h>

#define BIT_MASK(a, b) (size_t)(((size_t) -1 >> ((sizeof(size_t)*8) \
                                - (b))) & ~((1U << (a)) - 1))

unsigned address_to_set(size_t address) {
        return ((address & BIT_MASK(6, 12)) >> 6);
}

typedef struct item_t {
    double a1;
    double b2;
    double c3;
    double d4;
    double e5;
    double f6;
    double g7;
    double h8;
    int dummy; // padding
};


void mul()
{
    struct item_t item[1024];

    /*
     * 64 will make it jump to the next cache line in same
     * set when the dummy variable is not present
     *
     * */
    int i;
    for (i=0; i<1024; i+=64) {
        item[i].b2 = 23;
#ifdef DEBUG
        printf("%p\t%d\n", &(item[i]), address_to_set(&item[i]));
#endif
    }

;
    for (i=0; i<1024; i+=64) {
        item[i].h8 = 23;
#ifdef DEBUG
        printf("%p\t%d\n", &(item[i]), address_to_set(&item[i]));
#endif
    }

}


int main(void)
{
  mul();
}