C基础第22课--条件编译使用分析

学习自狄泰软件学院唐佐林老师C语言课程，文章中图片取自老师的PPT，仅用于个人笔记。

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实验1 ：条件编译是什么

实验2 : 通过命令行 定义宏

情景1 ：定义宏代表一个值

情景2：定义宏代表一个标识符是否存在。仅仅是定义一个宏，但是不给他赋值，宏和变量是不同的，宏可以代表一个值，他也可以仅仅表示一个标识符是否存在。

实验3：include 间接包含的时候

实验4; 使用条件编译解决 重复包含的编译错误

实验5 ：综合，使用条件编译定义产品的发布版和调试版

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实验1 ：条件编译是什么

// #include <stdio.h>

#define C 1

int main()
{
    const char* s;

    #if( C == 1 )
        s = "This is first printf...\n";
    #else
        s = "This is second printf...\n";
    #endif

    // printf("%s", s);
    
    return 0;
}



mhr@ubuntu:~/work/C$ gcc -E 22-1.c
# 1 "22-1.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "22-1.c"




int main()
{
    const char* s;


        s = "This is first printf...\n";






    return 0;
}
mhr@ubuntu:~/work/C$ 

实验2 : 通过命令行 定义宏

情景1 ：定义宏代表一个值
情景2：定义宏代表一个标识符是否存在。仅仅是定义一个宏，但是不给他赋值，宏和变量是不同的，宏可以代表一个值，他也可以仅仅表示一个标识符是否存在，

情景1 定义宏代表一个值

// #include <stdio.h>

int main()
{
    const char* s;

    #if( C == 1 )
        s = "This is first printf...\n";
    #else
        s = "This is second printf...\n";
    #endif

    printf("%s", s);
    
    return 0;
}


mhr@ubuntu:~/work/C$ gcc -DC=1 22-2.c
22-2.c: In function ‘main’:
22-2.c:13:5: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
     printf("%s", s);
     ^
22-2.c:13:5: warning: incompatible implicit declaration of built-in function ‘printf’
22-2.c:13:5: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
mhr@ubuntu:~/work/C$ 
mhr@ubuntu:~/work/C$ 
mhr@ubuntu:~/work/C$ ./a.out 
This is first printf...
mhr@ubuntu:~/work/C$ 

情景2：定义宏代表一个标识符是否存在。仅仅是定义一个宏，但是不给他赋值，宏和变量是不同的，宏可以代表一个值，他也可以仅仅表示一个标识符是否存在。

//#include <stdio.h>

int main()
{
    const char* s;

    #ifdef C
        s = "This is first printf...\n";
    #else
        s = "This is second printf...\n";
    #endif

    printf("%s", s);
    
    return 0;
}

单步编译 并通过命令行定义一个宏C

gcc -DC -E 22-2.c -o 22-2.i

# 1 "22-2.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "22-2.c"


int main()
{
    const char* s;


        s = "This is first printf...\n";




    printf("%s", s);

    return 0;
}

如果没有定义该宏 则会走另一个分支 ：gcc -E 22-2.c -o 22-2.i

# 1 "22-2.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "22-2.c"


int main()
{
    const char* s;




        s = "This is second printf...\n";


    printf("%s", s);

    return 0;
}

实验3：include 间接包含的时候

global.h

// global.h
int global = 10;

test.h

#include "global.h"
const char* NAME = "test.h";
char* hello_world(){  
  return "Hello world!\n";
  }

22-3.c

	// #include <stdio.h>
#include "test.h"
#include "global.h"

int main()
{
    const char* s = hello_world();
    int g = global;
    
    // printf("%s\n", NAME);
    // printf("%d\n", g);
    
    return 0;
}




mhr@ubuntu:~/work/C$ gcc 22-3.c
In file included from 22-3.c:2:0:
test.h:3:20: warning: extra tokens at end of #include directive
 #include "global.h"const char* NAME = "test.h";char* hello_world(){    return "Hello world!\n";}
                    ^
In file included from 22-3.c:3:0:
global.h:3:5: error: redefinition of ‘global’
 int global = 10;
     ^
In file included from test.h:3:0,
                 from 22-3.c:2:
global.h:3:5: note: previous definition of ‘global’ was here
 int global = 10;
     ^
22-3.c: In function ‘main’:
22-3.c:7:21: warning: implicit declaration of function ‘hello_world’ [-Wimplicit-function-declaration]
     const char* s = hello_world();
                     ^
22-3.c:7:21: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
mhr@ubuntu:~/work/C$ 

因为 global 重复定义了，因为重复间接的包含了同一个同文件，里面的 global 被重复的复制到 .c文件中，产生重复定义的错误。

gcc -E 22-3.c -o 22-3.i

# 1 "22-3.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "22-3.c"

# 1 "test.h" 1


# 1 "global.h" 1


int global = 10;
# 4 "test.h" 2
# 3 "22-3.c" 2
# 1 "global.h" 1


int global = 10;
# 4 "22-3.c" 2

int main()
{
    const char* s = hello_world();
    int g = global;




    return 0;
}

实验4; 使用条件编译解决 重复包含的编译错误

global.h

#ifndef _GLOBAL_H_
#define _GLOBAL_H_
int global = 10;

#endif

test.h

#ifndef _TEST_H_
#define _TEST_H_
#include "global.h"
const char* NAME = "test.h";
char* hello_world(){
    return "Hello world!\n";
    }

#endif

22-3.c

		// #include <stdio.h>
	#include "test.h"
	#include "global.h"
	
	int main()
	{
	    const char* s = hello_world();
	    int g = global;
	    
	    // printf("%s\n", NAME);
	    // printf("%d\n", g);
	    
	    return 0;
	}

编译通过没问题

实验5 ：综合

product.h

#define DEBUG 1
#define HIGH  1

.

#include <stdio.h>
#include "product.h"

#if DEBUG //是否定义了 DEBUG
    #define LOG(s) printf("[%s:%d] %s\n", __FILE__, __LINE__, s)
#else
    #define LOG(s) NULL
#endif

#if HIGH //是否定义了 HIGH
void f()
{
    printf("This is the high level product!\n");
}
#else
void f()
{
}
#endif

int main()
{
    LOG("Enter main() ...");
    
    f();
    
    printf("1. Query Information.\n");
    printf("2. Record Information.\n");
    printf("3. Delete Information.\n");
    
    #if HIGH
    printf("4. High Level Query.\n");
    printf("5. Mannul Service.\n");
    printf("6. Exit.\n");
    #else
    printf("4. Exit.\n");
    #endif
    
    LOG("Exit main() ...");
    
    return 0;
}



mhr@ubuntu:~/work/C$ gcc 22-4.c
mhr@ubuntu:~/work/C$ 
mhr@ubuntu:~/work/C$ ./a.out 
[22-4.c:23] Enter main() ...
This is the high level product!
1. Query Information.
2. Record Information.
3. Delete Information.
4. High Level Query.
5. Mannul Service.
6. Exit.
[22-4.c:39] Exit main() ...
mhr@ubuntu:~/work/C$