str.h

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

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

char* my_strchr(char *str, int chr)
{
    char *curchr = str;
    int i;
    size_t 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;
    size_t needle_len = strlen(needle);
    int matches = 0;
    char *begin = my_strchr(haystack, *needle);

    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(const char *str, size_t begin, size_t n)
{
    char *result;
    size_t len = strlen(str+begin);

    if (n < len) len = n;

    result = (char *) malloc(len + 1);
    assert(result != NULL);

    *(result+len) = '\0';
    return (char *) memcpy(result, str+begin, len);
}


int my_strsplit(char *str, char delimiter, size_t max_len, char *arr[])
{
    char *p = str;
    int start = 0;
    int n = 0;

    while(*p != '\0' && n < max_len)
    {
        if(*p == delimiter)
        {
            arr[n++] = my_substr(str, start, p-str-start);
            p++;
            start = p-str;
        }
        p++;
    }

    arr[n++] = my_substr(str, start, p-str);

    return n;
}