String search & replace in C

Question

I created a function to do that, and I want to know if there is a problem I'm overlooking about it ..

void replaceStr(buffer, haystack, needle, rep)
    char *buffer, *needle, *rep;
    const char *haystack;
{
    if (!buffer || !haystack || !needle || !rep)
        return;

    size_t rep_length = strlen(rep);
    long long diff = strlen(needle) - rep_length;
    memcpy(buffer, haystack, strlen(haystack) + 1);

    while ((buffer = strstr(buffer, needle))) {
        memcpy(buffer, rep, rep_length + 1); /* +1 for the null byte */
        if (diff > 0) {
            long long i;
            for (i = diff; i <= diff; ++i)
                buffer[i] = buffer[i+1];
            buffer[i] = 0;
        }
    }
}

Answer 1

Since this is about strings, I would rather use strcpy instead of memcpy. This has the benefit, that it saves the unnecessary strlen

strcpy(buffer, haystack);

You can declare the other parameters const too, e.g.

char *buffer;
const char *needle, *rep;

The for loop is a bit confusing, because it copies just one character at offset diff, no matter how large diff is. If you want to close a potential gap, you could again use strcpy

size_t needle_length = strlen(needle);
...
strcpy(buffer + rep_length, buffer + needle_length);

The other way round - if rep is larger than needle, you need to make space for the larger replacement string. Otherwise, you overwrite parts of haystack! (Unless this is what you intended, of course.)

---

You can avoid this, when you copy from haystack to buffer as you go, something like

size_t needle_length = strlen(needle);
char *found;
while ((found = strstr(haystack, needle))) {
    /* copy haystack part until needle */
    size_t n = found - haystack;
    strncpy(buffer, haystack, n);
    /* copy replacement string */
    strcpy(buffer + n, rep);
    /* adjust pointers */
    buffer += n + rep_length;
    haystack = found + needle_length;
}

/* copy remaining haystack */
strcpy(buffer, haystack);