C Project: Segmentation fault error

when i try to run C Project on linux ubuntu (12.04) i receive this erorr: Segmentation fault (core dumped) !! this is my code and his very sample please help

Segmentation fault (core dumped)

#include<stdio.h>
#include<string.h>
#define Z 0

/*
int my_strcmp(char *s1, char *s2);
int my_strcmp(char *s1, char *s2,int n);
*/

int main(int argc, char *argv[])
{
int num1=0;
char *str1;
char *str2;

printf("\n\nPlease select the number of the func that you want to use 
!\n\n");

printf("1)\tmy_strcmp\n2)\tmy_strncmp\n3)\tmy_strchar\n\n");
scanf("%d",&num1);

if(num1==1)
{
printf("\nplease enter the first string\t");
scanf("%s",str1);

printf("\nplease enter the second string\t");
scanf("%s",str2);
}

else if(num1==2)
{
printf("\nplease enter the first string\t");
scanf("%s",str1);

printf("\nplease enter the second string\t");
scanf("%s",str2);
}

else if(num1==3)
{
printf("\nplease enter the first string\t");
scanf("%s",str1);

printf("\nplease enter the second string\t");
scanf("%s",str2);
}

else printf("please entar the right number");

return 0;

}

i search on google to finde the problem but i don't fund