signal catching not work

The code is supposed to catch two segment fault, but the second is
always missing on my machine. Could anyone tell me why this code
doesn't work as expected?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <setjmp.h>

jmp_buf buf;

void sighandler(int a){
    static int i=0;
    longjmp(buf, ++i);
}

union {
    int *p;
    int i;
} a;

int *p;

int main(){
    if(signal(SIGSEGV, sighandler)==SIG_ERR){
        perror("error signal");
        abort();
    }
    switch(setjmp(buf)){
        case 0:
            a.i=0;
            printf("%d\n", *(a.p));
            break;
        case 1:
            puts("Structure way has failed to access value in position
0.");
            memset(&p,0,sizeof(int*));
            printf("%d\n", *p);
            break;
        default:
            puts("Memset way has failed to access value in position
0.");
    }
}
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