eclipse出現Symbol could not be resolved解決方案

• eclipse出現Symbol ‘NULL’ could not be resolved

借鑑老外解決方案：順便吐槽一下百度垃圾

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：Here are the details if anybody else has this problem:

I could not clear the settings from the Entries tab, but your suggestion got me to
 Project Properties > C/C++ General > Preprocessor Include Paths > Providers (tab). 
with this information: "Options of global providers below can be changed in (linked->) Workspace Settings, Discovery Tab."

Clicking the link brought me to a dialog I could have reached directly with Eclipse 
Menu > Window > Preferences > C/C++ > Build > Settings > Discovery (tab). 
I selected "CDT GCC Built-in Compiler Settings MinGW [ Shared ]",
 and pressed the Clear Entries button.
Then I pressed the Apply button (OK probably would have worked, too), and I was good to go.

Thanks again

本人借鑑：沒有選擇MinGW，直接選擇CDT GCC Built-in Compiler Settings [ Shared ]問題解決。