# include <h-char-sequence> new-line searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header.
How the places are specified or the header identified is implementation-defined.
以下節選自ISO/IEC 9899:1999原文：
注意到最後了嗎，implementation-defined——沒人規定說就是系統目錄，而且很多編譯器的預設行為也不是只找系統目錄。即使是同一種編譯器，具體的行為尚且可能取決於你提供的選項呢，何況C語言編譯器五花八門。