关于头文件定义方法的分析

C 预处理

本调研分析1~5由李子天于2015.11.29提供,后续由张昱补充。

1.gcc file1.c file2.c

• head.h:
  short int a = 10;
• file1.c:
  #include "head.h"
  main(){}
• file2.c:
  #include "head.h"

链接器报错：a重定义

2.gcc file1.c file2.c

• head.h:
  #ifndef _HEAD_H
  #define _HEAD_H
  short int a = 10;
  #endif
• file1.c:
  #include "head.h"
  main(){}
• file2.c:
  #include "head.h"

链接器报错：a重定义

3.gcc file1.c

• head.h:
  short int a = 10;
• file1.c
  #include "head.h"
  #include "head.h"
  main(){}

编译器报错：a重定义

4.gcc file1.c

• head.h:
  #ifndef _HEAD_H
  #define _HEAD_H
  short int a = 10;
  #endif
• file1.c
  #include "head.h"
  #include "head.h"
  main(){}

正常编译运行。

5.gcc file1.c file2.c file3.c

• head.h:
  extern short int a;
• file1.c:
  #include "head.h"
  main(){}
• file2.c:
  #include "head.h"
• file3.c:
  short int a = 10;

正常编译运行。

6.gcc file1.c file3.c

• head.h:
  short int a = 10;
• file1.c:
  #include "head.h"
  main(){}
• file3.c:
  #ifndef _HEAD_H
  #define _HEAD_H
  #include "head.h"
  #endif

链接器报错：a重定义

7.gcc file1.c file3.c

• head.h:
  #ifndef _HEAD_H
  #define _HEAD_H
  short int a = 10;
  #endif
• file1.c:
  #include "head.h"
  main(){}
• file3.c:
  #ifndef _HEAD_H
  #define _HEAD_H
  #include "head.h"
  #endif

正常编译运行。