1. gcc在哪里查找.h文件 https://askubuntu.com/questions/573417/where-are-header-files-for-gcc-located

  2. 即使按照上面的办法找到了一堆的目录，多个目录下有系统的.h文件，到底是哪个呢？ https://stackoverflow.com/questions/13079650/how-can-i-find-the-header-files-of-the-c-programming-language-in-linux https://unix.stackexchange.com/questions/1697/how-can-i-know-which-unistd-h-file-is-loaded

  3. 如何查找某个函数是属于哪个.h文件的呢？ https://stackoverflow.com/questions/2375199/how-to-find-the-header-file-to-be-included-for-a-library-function-in-linux 比如 系统调用的write man 2 write