I need to write a program that takes two strings as arguments and check if the second one is a substring of the first one. I need to do it without using any special library functions. I created this implementation, but I think it's always returning true as long as there is one letter that's the same in both strings. Can you help me out here. I'm not sure what I am doing wrong:

#include <stdio.h>
#include <string.h>

int my_strstr( char const *s, char const *sub ) {
    char const *ret = sub;

    int r = 0;
    while ( ret = strchr( ret, *sub ) ) {
        if ( strcmp( ++ret, sub+1 ) == 0 ){
            r = 1;
        }
        else{
            r = 0;
        }        
    }
    return r;
}

int main(int argc, char **argv){

    if (argc != 3) {
        printf ("Usage: check <string one> <string two>\n");
    }
    int result = my_strstr(argv[1], argv[2]);

    if(result == 1){
        printf("%s is a substring of %s\n", argv[2], argv[1]);
    } else{
        printf("%s is not a substring of %s\n", argv[2], argv[1]);
    }
    return 0;
}