#include <stdio.h>

const char *find_a_substring(const char *string, const char *substring)
{
    if (string == NULL || substring == NULL ||
       *substring == 0 || *string == 0) return NULL;
    for(const char * t = substring;*string;++string, t = substring)
    {
        for(const char *s = string; *s && *t && *t == *s; ++s, ++t);
        if (*t == 0) return string;
    }
    return NULL;
}

int main(int argc, const char * argv[])
{
    printf("%s\n",find_a_substring("Hello world!","o w"));
    printf("%s\n",find_a_substring("Hello world!","H"));
    printf("%s\n",find_a_substring("Hello world!","!"));
    printf("%s\n",find_a_substring("Hello world!","world!!!"));
}