size_t BUF_MEM_grow(BUF_MEM *str, size_t len)
{
    char *ret;
    size_t n;

    if (str->length >= len) {
        str->length = len;
        return (len);
    }
    if (str->max >= len) {
        if (str->data != NULL)
            memset(&str->data[str->length], 0, len - str->length);
        str->length = len;
        return (len);
    }
    /* This limit is sufficient to ensure (len+3)/3*4 < 2**31 */
    if (len > LIMIT_BEFORE_EXPANSION) {
        BUFerr(BUF_F_BUF_MEM_GROW, ERR_R_MALLOC_FAILURE);
        return 0;
    }
    n = (len + 3) / 3 * 4;
    if ((str->flags & BUF_MEM_FLAG_SECURE))
        ret = sec_alloc_realloc(str, n);
    else
        ret = OPENSSL_realloc(str->data, n);
    if (ret == NULL) {
        BUFerr(BUF_F_BUF_MEM_GROW, ERR_R_MALLOC_FAILURE);
        len = 0;
    } else {
        str->data = ret;
        str->max = n;
        memset(&str->data[str->length], 0, len - str->length);
        str->length = len;
    }
    return (len);
}