How to config YouCompleteMe for C language?

Recently, I installed YouCompleteMe and I'm new user in it. I read installation instructions from this link: http://valloric.github.io/YouCompleteMe. Now, I can use it for python and etc, but I can't use it for C. I don't understand what's the meaning of this sentence from the link: "forget that if you want the C-family semantic completion engine to work, you will need to provide the compilation flags for your project to YCM." I read the YCM user guide, but I couldn't solve my problem. Tnx from anybody who explains me what is the meaning of the above sentence and what do I do?

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