Recreating strstr()

Question

My goal is to recreate strstr in a C function named myStrStr that returns a 1 if the substring is found in the haystack and a 0 if it is not, all while copying the matched substring into a buffer.

I tried writing the following code to solve this, and I'm unsure of why it doesn't produce the desired result when it is compiled and run.

int myStrStr(char *haystack, char *needle, char *buffer) {
    int i = 1;
    char *k = (haystack + i);
    for (; *haystack != '\0'; haystack++) {
        if (*haystack == *needle) {
            buffer[0] = *haystack;
            for (; *needle != '\0'; needle++) {
                if (*needle = *k) {
                    buffer[i] = *k;
                    i++;
                    printf("%d\n", i);
                } else
                if (strlen(buffer) == strlen(needle)) {
                    return 1;
                } else {
                    buffer[0] = 0;
                }
            }
        }
    }
    return 0;
}

an example of the driver code is as follows:

int myStrStr(char[], char[], char[]);

char haystack[][20] = { "chocolate", "vanilla", "caramel", "strawberry", "banana", "cherry" };
    char needle[][20] = { "choc", "lla", "am", "strawberry", "na", "terrible" };
    char buffer[255];

printf("\n\t=========Test #1: myStrStr with '%s' and substring '%s'===========\n\n", haystack[0], needle[0]);
int result = myStrStr(haystack[0],needle[0],buffer);
assert(result == 1&& strcmp(needle[0], buffer) == 0);
printf("\n\t\t....Test Passed\n");

printf("\n\t=========Test #2: myStrStr with '%s' and substring '%s'===========\n\n", haystack[1], needle[1]);
result = myStrStr(haystack[1],needle[1],buffer);
assert(result == 1 && strcmp(needle[1], buffer) == 0);
printf("\n\t\t....Test Passed\n");

Thanks

Answer 1

The function is broken in multiple ways.

Here is a corrected version:

int myStrStr(const char *haystack, const char *needle, char *buffer) {
    if (*needle == '\0') {
        /* special case for empty needle: return success with an empty match */
        buffer[0] = '\0';
        return 1;
    }
    for (; *haystack != '\0'; haystack++) {
        if (*haystack == *needle) {
            buffer[0] = *haystack;
            for (size_t i = 1;; i++) {
                buffer[i] = needle[i];
                if (needle[i] == '\0')
                    return 1;
                if (needle[i] != haystack[i])
                    break;
            }
        }
    }
    buffer[0] = '\0';
    return 0;
}

Note that copying the matched string while you scan is unnecessary, it would be sufficient to copy the needle once a match is found. As a matter of fact, the contents of buffer is either an empty string or a copy of needle : not a very useful side effect.