C++ Essay

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
 *
 *************************************************/

https://en.cppreference.com/w/c/language/basic_concepts 裡提到
A C program is a sequence of text files (typically header and source files) that contain declarations. They undergo translation to become an executable program, which is executed when the OS calls its main function (unless it is itself the OS or another freestanding program, in which case the entry point is implementation-defined).

freestanding program或是stand-alone program，本身的程式不依靠外部的模組， 通常用在bootstrap loading，因為不依靠外部模組，loader很簡單就直接指到要執行的指令開始跑

在C99規範中，§5.1.2定義了執行環境Execution environments – freestanding、 hosted environment

其中有個重點:
All objects with static storage duration shall be initialized (set to their
initial values) before program startup

Freestanding定義在C99 §5.1.2.1

§5.1.2.1 Freestanding environment In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.

描述例如program startup的entry point是implementation defined，相對於我們一般熟知的main(定義在hosted environment §5.1.2.2.1裡)