參考 https://en.cppreference.com/w/c/comment整理一些注意的重點
All comments are removed from the program at translation phase 3 by replacing each comment with a single whitespace character.
這裡描述在translation phase3對於comment做的事，用一個space character取代
translation phase 3可參考C99 §5.1.1.2 (p.10)
3. The source file is decomposed into preprocessing tokens6) and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or in a partial comment. Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character is implementation-defined
Except within a character constant, a string literal, or a comment, the characters /* introduce a comment.
這邊解釋為什麼 嵌套 /* /* */ */ 會不對稱的原因，因為第二個 /* 已經在 comment內，所以第一個/* 會 match 倒數第二個*/，而留下最後一個不對稱的 */
關於為什麼語言設計時不設計成支援嵌套的原因可參考，http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf 6.4.9
主要的原因是C89的committee認為有其他方式可以處理nested comment，也可參考 comment tips這邊的說明
像上面這種註釋法採用conditional directive有時也被稱為conditioned out，要注意的是 c comment /* */ 的取代還有preprocessing token解析等是在執行preprocessor前進行，所以這邊的#if 0, #endif 的內容一開始也會被處理。
其他有關comment的基本常識
http://www.jargon.net/jargonfile/w/wingedcomments.html
winged comments: 單行 /* */ 註解，相對於boxed comments
/*************************************************
*
* This is a boxed comment in C style
*
*************************************************/