GCC與.h文件

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

即使按照上面的辦法找到了一堆的目錄，多個目錄下有系統的.h文件，到底是哪個呢？
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

如何查找某個函數是屬於哪個.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 系統調用的write
man 2 write