I have trouble in plugin.

I want to learn how to write plugin.
I had read some websites,which tell me I should use the Qt and add some head files.But I could not find this head files on my Linux computer.
Could you tell me how to find the head files or how to start it?
Thanks!

First you should be able to build LibreCAD from source. There is a wiki article about it on https://wiki.librecad.org/

When you have successfully build LibreCAD and are able to run your own build application you're ready to start with plugin.
Go to the plugins folder in LibreCAD source, there is everything you need.
Examine an existing plugin, e.g. sample plugin and understand how it works.
Then copy a plugin which is similar to that you want to write and work on that copy.

Armin

Thanks for your replay!I will try it.

I had done all of this,and I get some ".so" files .
But I don't know how to use Plugins.
could you help me?
Thanks!

The make system should have put them to the right location already.
On Linux they are copied to [YOUR LIBRECAD PATH]/unix/resources/plugins.
And when you run unix/librecad they are loaded automatically.
Then you find your plugin in the Plugins menu, with the name you gave it in source code.