为什么这个UDP客户端/服务器代码不起作用？

I'm experimenting with trying to send a single UDP message from one machine to another. The client is an embedded Linux system that's connected to the internet via a modem. No firewall is in place. The server is a public VPS server I have. Despite calling the client a number of times the server never receives anything. Can anyone see what could be going wrong?I'm experimenting with trying to send a single