problem with the func 'strstr'

Hello, im using the func 'strstr' and for some reason it doesnt return NULL even though the two strings are completely diffrent, here is my work:

#include<stdio.h> #include<conio.h> #include<string.h> #include<malloc.h> #define N 100 void replaceSubstring(char *str, char *sub_str); int main() { int flag=1,flag2=1; char str[N],sub_str[N],i; char *pos,*pos2; while(flag) { printf("Enter text :
"); gets(str); if(str[0]==0) { flag=0; printf(""); free(str); free(sub_str); getch(); return 0; } else { printf(" substring :
"); gets(sub_str); replaceSubstring(str,sub_str); } } } void replaceSubstring(char *str, char *sub_str) { char *pos; int flag2=1,i; while(flag2) { pos=strstr(str,sub_str); if(pos != NULL) { for(i=0;sub_str[i]>0;i++) { pos[i]-=32; } } else { flag2==0; } } }