str.h

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

char my_charat(char *str, size_t i)
{
    return str[i];
}

char* my_strchr(char *str, int chr)
{
    char *curchr = str;
    int i;
    int len = strlen(str);

    for(i = 0; i < len; i++)
    {
        if(chr == my_charat(curchr, i)){
            return (char *)(curchr+i);
        }
    }

    return NULL;
}

char* my_strstr(char *haystack, char *needle)
{
    int i;
    int needle_len = strlen(needle);
    int matches = 0;
    char *begin = my_strchr(haystack, my_charat(needle, 0));

    while(!matches && begin != NULL)
    {
        matches = 1;
        for(i = 1; i < needle_len && matches; i++)
        {
            if(my_charat(begin, i) != my_charat(needle, i))
            {
                matches = 0;
                begin = my_strchr(begin+1, my_charat(needle, 0));
            }
        }
    }

    return begin;
}

char* my_substr(char *str, size_t begin, size_t len)
{
    return strndup(str+begin, len);
}