Preprocessor 預處理器

• 在編譯器將您的源代碼編譯為組語前，還有一個稱為預處理的附加階段。
• Objective: Produce a source code file with preprocessing commands properly sorted out.
• Preprocessor commands are known as Directives.
• Preprocessor provides certain features, which are called preprocessor directives.
• Preprocessor directive 以 # 為開頭。(例如：#include <stdio.h>)
• Preprocessor directives 可以被放在程式中的任何地方。但建議放在開頭，並且每個必須獨立一行。

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#include <stdio.h>
#define MACRO_DEMO 123
int main() {
    printf("demo:␣%d\n", MACRO_DEMO);
    return 0;
}

• 以 gcc -E main.c 查看預處理後的狀態。

Macro 巨集

• #define is known as macro expansion.
• General Form: #define macro_template macro_expansion

Macro Expansion

• Preprocessor searches for macro definition.
• After finding macro definition, it searches the whole program for macro_template.
• Replace every macro_template with macro_expansion.
  • Replacement will not occur if the template is in a quoted string.
• Notes:
  • 通常 macro_template 會使用大寫字母
  • 不要使用分號 ;

• 為什麼要用巨集?
  • 為了寫出更高效率的程式。
  • 提高程式的可讀性。
  • 定義的巨集名稱可作為其他定義的一部分。
• 巨集可以像函式一樣擁有 arguments。
• Notes：
  • 在定義巨集時，參數和名稱之間不允許有空格。
  • Macro expansions 應該用括號括起來。
  • 使用 \ 將巨集拆分為多行。

Macro vs. Function

Macro

• 單純替換
• 雖然你可能沒感覺，但比函式更快。

Function

• 傳遞參數 -> 執行 -> 回傳結果
• 支援遞迴

inline Functions

• inline is a CPP keyword.
• C includes this keyword from C99. Though GNU C (and some other compilers) had inline functions long before standard C.
• Comparison:
  1. inline is just a suggestion.
  2. inline will check types.
• I will not talk too much about this keyword since it is different from CPP. I do not want to mislead you.
• If you are interested in this topic, please read here.

File Inclusion

• Why File Inclusion?
  • Divide a program in multiple files.
    • Each file contains related functions.
    • How to classify functions. Up to you.
  • Some functions or macros are required in lot of programs.
    • Put them in a file. Make them a library.
    • Include them when you need them.
• What does Inclusion Mean?
  • Nothing but simply copy and paste.
  • Nested Inclusion is supported.

Conditional Compilation

• Write one code to run on different environments.
  • #if: if.
  • #else: else.
  • #elif: else if.
  • #endif: end if.
  • #ifdef: if defined.
  • #ifndef: if not defined.

ifdef and endif

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#ifdef macroname
    statement sequence.
#endif

• 如果 macroname 被定義了，那麼 #ifdef 和 #endif 之間的程式將被執行。
• 範例：

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  #ifndef TEST_H #define TEST_H ... #endif

  • 但也可用 #pragma once

• gcc -D OS_LINUX os_dependent.c
• -D: Predefine name as a macro, with definition 1.
• So one code can be executed on different OSs. What you need to do is to build your code with different definitions.
• This is a very useful technique. Let’s see a real case (cJSON).

• 請自行去了解 # 及 ## 在巨集中的作用。