st3.c the C puzzle book

st3.c

#include "defs.h"

struct S1 {
        char *s;
        struct S1 *s1p;
};

void swap(struct S1 *p1, struct S1 *p2)
{
        char *temp = p1->s;
        p1->s = p2->s;
        p2->s = temp;
}


int main()
{
        static struct S1 a[] = {
                { "abcd", a+1 },
                { "efgh", a+2 },
                { "ijkl", a+3 }
        };

        struct S1 *p[3];
        int i;

        for( i=0; i<3; i++) p[i] = a[i].s1p;
        PRINT3(s, p[0]->s, (*p)->s, (**p).s);

        swap(*p, a);
        PRINT3(s, p[0]->s, (*p)->s, (*p)->s1p->s);

        swap(p[0], p[0]->s1p);
        PRINT3(s, p[0]->s, (*++p[0]).s, ++(*++(*p)->s1p).s);
}

p[0]->s = efgh  (*p)->s = efgh  (**p).s = efgh
p[0]->s = abcd  (*p)->s = abcd  (*p)->s1p->s = ijkl
Segmentation fault (core dumped)