if (c == csv->sep_char || is_SEPX (c)) {

##</code><code>##

/* somewhere on the object constructor */
csv->sep_tail_len = sep_len - 1;
csv->sep_tail = sep + 1;
csv->sep_char = *sep;
...

/* then, on the parser */
if (c == csv->sep_char) {
    if (!csv->sep_tail_len ||
        ((csv->size - csv->used >= csv->sep_tail_len) &&
         !memcmp(csv->bptr + csv->used, csv->sep_tail, csv->sep_tail_len))) {
        /* you have a separator! */