callBack

the error in below program is segmentation fault.
please can you suggest me how to resolve this issue. and is it correct method for a call back in c .
Thanks and regards,
K Subrahmaniam
#include <stdio.h>
#include<unistd.h>

typedef void (*CANFD_DataAppCallBack)( int reason);

typedef struct temp
{
CANFD_DataAppCallBack callBack;
} x;

static void mcancallBack(int reason)
{
printf("reason is %d\n",reason);
if(reason==0)
printf("This is callback\n");
}

int reason;
x *ptr;
int main()
{
reason =0;
ptr->callBack=mcancallBack;
return 0;
}