private function readNumber()
    {
        // Open
        if (false === $this->handle = fopen($this->filePath, 'c+')) {
            throw new RuntimeException("Failed to open file {$this->filePath}.");
        }
        // Exclusive lock
        if (!flock($this->handle, LOCK_EX)) {
            throw new RuntimeException("Failed to lock file {$this->filePath}.");
        }

        return fread($this->handle, $this->length);
    }