static void
lock_dbg_cb(int mode, int type, const char *file, int line)
{
    static int modes[CRYPTO_NUM_LOCKS]; /* = {0, 0, ... } */
    const char *errstr = NULL;
    int rw;

    rw = mode & (CRYPTO_READ | CRYPTO_WRITE);
    if (!((rw == CRYPTO_READ) || (rw == CRYPTO_WRITE))) {
        errstr = "invalid mode";
        goto err;
    }
    if (type < 0 || type >= CRYPTO_NUM_LOCKS) {
        errstr = "type out of bounds";
        goto err;
    }
    if (mode & CRYPTO_LOCK) {
        if (modes[type]) {
            errstr = "already locked";
            /*
             * must not happen in a single-threaded program
             * (would deadlock)
             */
            goto err;
        }
        modes[type] = rw;
    } else if (mode & CRYPTO_UNLOCK) {
        if (!modes[type]) {
            errstr = "not locked";
            goto err;
        }
        if (modes[type] != rw) {
            errstr = (rw == CRYPTO_READ) ?
                "CRYPTO_r_unlock on write lock" :
                "CRYPTO_w_unlock on read lock";
        }
        modes[type] = 0;
    } else {
        errstr = "invalid mode";
        goto err;
    }

err:
    if (errstr) {
        /* we cannot use bio_err here */
        fprintf(stderr, "openssl (lock_dbg_cb): %s (mode=%d, type=%d) at %s:%d\n",
            errstr, mode, type, file, line);
    }
}