--- tags: DYKC, CLANG, C LANGUAGE --- # [你所不知道的 C 語言](https://hackmd.io/@sysprog/c-prog/): 開發工具和規格標準 Copyright (**慣C**) 2015, 2017 [宅色夫](http://wiki.csie.ncku.edu.tw/User/jserv) ==[直播錄影](https://www.youtube.com/watch?v=scLFY2CRtFo)== :::success "If I had eight hours to chop down a tree, I’d spend six hours sharpening my axe." -- Abraham Lincoln > 「如果我有 8 小時可以砍 1 棵樹,我會花 6 小時把斧頭磨利。」(類似漢語「工欲善其事,必先利其器」的精神) -- 亞伯拉罕.林肯 語言規格: C89/[C90](https://www.iso.org/standard/17782.html) -> [C99](https://www.iso.org/standard/29237.html) -> [C11](https://www.iso.org/standard/57853.html) -> C17/[C18](https://www.iso.org/standard/74528.html) -> C2x ::: ## C 語言老爸的評論 「C 很彆扭又缺陷重重,卻異常成功。固然有歷史的巧合推波助瀾,可也的確是因為它能滿足於系統軟體實作的程式語言期待:既有相當的效率來取代組合語言,又可充分達到抽象且流暢,能用於描述在多樣環境的演算法。」 > C is quirky, flawed, and an enormous success. Although accidents of history surely helped, it evidently satisfied a need for a system implementation language efficient enough to displace assembly language, yet sufficiently abstract and fluent to describe algorithms and interactions in a wide variety of environments. —— [Dennis M. Ritchie](https://en.wikipedia.org/wiki/Dennis_Ritchie) ![](https://i.imgur.com/1gWHzfd.png) [Jonathan Adamczewski](https://twitter.com/twoscomplement) 貼出經典著作《The C Programming Language》,然後評註說: > "**C++: The Good Parts**" ![](https://i.imgur.com/4Uzmkvi.png) C++ 可以美得令人不知所措 [[source](https://twitter.com/jfbastien/status/730963193799938051)] ## 為什麼我不探討 C++ * 在台灣發文好像愛用「為什麼我不」開頭,後面可接「念研究所」、「待在大公司」等描述 * C++ 自稱為物件導向的程式語言,卻不願意對物件在執行時期的表現負責任 * 若說 C 語言給了你足夠的繩子吊死自己,那麼 C++ 給的繩子除了夠你上吊之外,還夠綁住你身邊的朋友 * 相較之下,Java 讓你在吊死自己之際仍有親友監視著,雖然死不了,但事後會更想死 * [ [source](https://twitter.com/RichRogersHDS/status/666798359244611584) ] * In Ruby, everything is an object. * In Clojure, everything is a list. * In Javascript, everything is a terrible mistake. * in C, everything is a representation (unsigned char [sizeof(TYPE)]). * Linus Torvalds [在 2010 年的解釋](http://www.realworldtech.com/forum/?threadid=104196&curpostid=104208) * C++ 實際上已經是截然不同的程式語言 * C++ 老爸 Bjarne Stroustrup 的文章: "[Learning Standard C++ as a New Language](http://www.stroustrup.com/new_learning.pdf)" * 最重要的是,C++ 改版飛快,C++ 17 即將推出,但我還沒看懂 C++ 98 ![](https://i.imgur.com/ITVm6gI.png) * [ [source](https://isocpp.org/std/status) ] ## 延伸閱讀 * [沒有 C 語言之父,就沒有 Steve Jobs](https://buzzorange.com/techorange/2015/10/19/without-dennis-ritchie-there-would-be-no-steve-jobs/) ([原文](https://www.zdnet.com/article/without-dennis-ritchie-there-would-be-no-jobs/)) * [第一個 C 語言編譯器是怎樣編寫的?](https://kknews.cc/zh-tw/tech/bx2r3j.html) <img style="display:block; margin:auto;" src="https://i.imgur.com/GMpwC60.png"></img> ## 讀規格書可大幅省去臆測 在 [ISO/IEC 9899 (a.k.a C99 Standard)](http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf) 中 5.1.2.2.1 內有提到 C Standard 要求 main 函數必須這樣寫 ```clike int main(void) { /* ... */}; ``` 或者: ```Clike int main(int argc, char *argv[]) { /* ... */ }; ``` C++ 之父 Bjarne Stroustrup 的個人網頁內有個 [FAQ](http://www.stroustrup.com/bs_faq2.html#void-main) 裡面有個問題叫 **Can I write "void main()"?** 然而在 C++ 與 C 的標準中從來沒出現過這樣的寫法,也就是說,```void main()``` 這個寫法從來沒正確過 延伸閱讀 * [C 語言中 int main() 和 void main() 有何區別?](https://www.zhihu.com/question/60047465) * [ C++ 的 void main() / int main() … 不要再用 void main() 了! | Peter Dave Hello's Blog](https://www.peterdavehello.org/2014/10/void-main-int-main-in-c-and-cpp/) * [void main(void) - the Wrong Thing](https://www.ty-penguin.org.uk/~auj/voidmain/) --- ## ISO/IEC 9899 (簡稱 "C99") - 從[一則笑話](https://twitter.com/SoManyHs/status/675505383008415744)談起 - "Programming in C: if it doesn't work, just add a star. Or multiple stars. Or ampersands." - 葉秉哲博士的[推文](https://twitter.com/william_yeh/status/705031736371982336):「==溯源能力==是很重要的,才不會被狀似革新,實則舊瓶裝新酒或跨領域借用的『新觀念』所迷惑」 - [規格書](http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf) (PDF) 搜尋 "***object***",共出現 735 處 - 搜尋 "***pointer***",共出現 637 處。有趣的是,許多教材往往不談 object,而是急著談論 pointer,殊不知,這兩者其實就是一體兩面 - object != object-oriented - 前者的重點在於「資料表達法」,後者的重點在於 "everything is object" - C11 ([ISO/IEC 9899:201x](http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1570.pdf)) / [網頁版](http://port70.net/~nsz/c/c11/n1570.html) - 從第一手資料學習:大文豪寫作都不免要查字典,庸俗的軟體開發者如我們,難道不需要翻閱語言規格書嗎?難道不需要搞懂術語定義和規範嗎? - `&` 不要都念成 and,涉及指標操作的時候,要讀為 "address of" - C99 標準 [6.5.3.2] Address and indirection operators 提到 '==&==' address-of operator - C99 [3.14] ***object*** - region of data storage in the execution environment, the contents of which can represent values - 在 C 語言的物件就指在執行時期,==資料==儲存的區域,可以明確表示數值的內容 - 很多人誤認在 C 語言程式中,(int) 7 和 (float) 7.0 是等價的,其實以資料表示的角度來看,這兩者截然不同,前者對應到二進位的 "111",而後者以 IEEE 754 表示則大異於 "111" <img style="display:block; margin:auto;" src="https://i.imgur.com/BkxFbFh.png"></img> - A pointer to void shall have the same representation and alignment requirements as a pointer to a character type. > 關鍵描述!規範 `void *` 和 `char *` 彼此可互換的表示法 ```clike void *memcpy(void *dest, const void *src, size_t n); ``` - C99 規格書的解說就比很多書本清楚,何必捨近求遠呢? - **EXAMPLE 1**: The type designated as `float *` has type "pointer to float". Its type category is pointer, not a floating type. The const-qualified version of this type is designated as `float * const` whereas the type designated as "`const float *` is not a qualified type — its type is "pointer to const qualified float" and is a pointer to a qualified type. - **EXAMPLE 2**: The type designated as "`struct tag (*[5])(float)` has type "array of pointer to function returning struct tag". The array has length five and the function has a single parameter of type float. Its type category is array. - [Understand more about C](https://www.slideshare.net/YiHsiuHsu/understand-more-about-c) 提及若干肇因於不同的 C 語言標準,而使得程式碼行為不同的案例 ### 規格不能只看新的,過往也要熟悉 ![](https://i.imgur.com/oerJv9s.png) [source](https://twitter.com/0xdeadb/status/766293771663339520) - 空中巴士 330 客機的娛樂系統裡頭執行 14 年前的 Red Hat Linux,總有人要為「古董」負責 - 而且空中巴士 380 客機[也是如此](https://twitter.com/AlxRogan/status/766382294038872064) 為何 C 語言標準函式庫裡頭的函式名稱如此簡短?像是 * strcpy * strlen 最初連結器有 6 到 8 個字元的輸入限制! > Translation limits > 6 significant initial characters in an external identifier * 延伸閱讀: * [Why did ANSI only specify six characters for the minimum number of significant characters in an external identifier?](https://stackoverflow.com/questions/38035628/c-why-did-ansi-only-specify-six-characters-for-the-minimum-number-of-significa/38042724#38042724) * [Identifier](https://en.cppreference.com/w/c/language/identifier) [The Design and Evolution of C++](http://www.stroustrup.com/dne.html) 一書對應的解說: ![](https://i.imgur.com/rDNRibU.jpg) ## 英文很重要 安裝 `cdecl` 程式,可以幫你產生 C 程式的宣告。 ```shell $ sudo apt-get install cdecl ``` 使用案例 ```shell $ cdecl cdecl> declare a as array of pointer to function returning pointer to function returning pointer to char ``` 會得到以下輸出: ```clike char *(*(*a[])())() ``` 把前述 C99 規格的描述帶入,可得: ```shell cdecl> declare array of pointer to function returning struct tag ``` ```clike struct tag (*var[])() ``` 如果你沒辦法用英文來解說 C 程式的宣告,通常表示你不理解! `cdecl` 可以解釋 C 程式宣告的意義,比方說: ```shell cdecl> explain char *(*fptab[])(int) declare fptab as array of pointer to function (int) returning pointer to char ``` --- ## 只用 printf 觀察資料,有問題嗎? ![](https://i.imgur.com/5pwv9bT.png) [ [source](https://twitter.com/alang198611/status/735312419715612673) ] - 只用 `printf()` 觀察的話,永遠只看到你設定的框架 (format string) 以內的資料,但很容易就忽略資料是否合法、範圍是否正確,以及是否看對地方 - `printf()` 大概是最早被記下來的函式,也困擾很多人,有意思的是,1960 年代初期 MIT 開發的 [CTSS 作業系統](https://en.wikipedia.org/wiki/Compatible_Time-Sharing_System) 中,終端機命令就包含了 printf,後者一路從 Multics 和 Unix 繼承至今 - 在 CTSS 原始程式碼的檔案 com3 中可見到這行 `STMTDC PRINTF,11,T,T25`,前一行註解寫 "The following tables are the dictionaries of statement types" ## 不要急著印出位址,善用 GDB ![](https://hackpad-attachments.s3.amazonaws.com/embedded2015.hackpad.com_s0rlzR8wVtm_p.299401_1474178146663_undefined) [source](https://twitter.com/noreenahertz/status/593761269930434560): NASA before PowerPoint, 1961 - 「學會了 GDB,我有種山頂洞人==學會用火==的感動」 -- 張至 - [GDB Rocks!](http://www.slideshare.net/chenkaie/gdb-rocks-16951548) - [Introduction to gdb](http://www.slideshare.net/owenhsu/introduction-to-gdb-3790833) - [Debugging with GDB](http://www.slideshare.net/linaroorg/connect12-q2-gdb) - [除錯程式: gdb](https://www.cyut.edu.tw/~ckhung/b/c/gdb.php) - [Introduction to GDB a tutorial - Harvard CS50](https://www.youtube.com/watch?v=sCtY--xRUyI) (教學影片) - [透過 GDB 學習 C 語言](https://jasonblog.github.io/note/gdb/tongguo_gdb_xue_xi_c_yu_yan.html) ## GDB * [Kernel command using Linux system calls](https://developer.ibm.com/tutorials/l-system-calls/) * video: [Linux basic anti-debug](https://www.youtube.com/watch?v=UTVp4jpJoyc) * video: [C Programming, Disassembly, Debugging, Linux, GDB](https://www.youtube.com/watch?v=twxEVeDceGw) * [rr](http://rr-project.org/) (Record and Replay Framework) * video: [Quick demo](https://www.youtube.com/watch?v=hYsLBcTX00I) * video: [Record and replay debugging with "rr"](https://www.youtube.com/watch?v=ytNlefY8PIE) --- ## 除了 Vim,我推薦 Visual Studio Code * Microsoft 開放原始碼專案 Visual Studio Code (VS Code) * video: [五個 Visual Studio Code 的實用工具與技巧](https://www.youtube.com/watch?v=zzon9KS90Dk) * [共筆](https://hackmd.io/s/rJPKpohsx)