C語言進階：23、#error和#line的用法

#error用於生成一個編譯錯誤消息

用法：

		#error message
		message不需要用雙引號包圍

#error編譯指示字用於自定義程序員特有的編譯錯誤消息，類似的，#warning用於生成編譯警告。

#error是一種預編譯指示字，可用於提示編譯條件是否滿足

#ifndef __cpluscplus //C++內置的宏   通過檢測這個宏的存在，來進行錯誤提醒。
#error This file should be processed with C++ compiler
#endif

編譯過程中任意錯誤信息意味着無法生成最終的可執行程序。

#include <stdio.h>

void f()
{
#if ( PRODUCT == 1 )
    printf("This is a low level product!\n");
#elif ( PRODUCT == 2 )
    printf("This is a middle level product!\n");
#elif ( PRODUCT == 3 )
    printf("This is a high level product!\n");
#else
    #warning The macro Product is NOT defined! //#error The macro Product is NOT defined!
#endif
}

int main()
{
    f();
    
    printf("1. Query Information.\n");
    printf("2. Record Information.\n");
    printf("3. Delete Information.\n");

#if ( PRODUCT == 1 )
    printf("4. Exit.\n");
#elif ( PRODUCT == 2 )
    printf("4. High Level Query.\n");
    printf("5. Exit.\n");
#elif ( PRODUCT == 3 )
    printf("4. High Level Query.\n");
    printf("5. Mannul Service.\n");
    printf("6. Exit.\n");
#endif
    
    return 0;
}

運行輸出：

~/will$ gcc 23-2.c
23-2.c: In function ‘f’:
23-2.c:12: error: #error The macro Product is NOT defined!

將#error修改爲warnning：

~/will$ gcc 23-2.c
23-2.c: In function ‘f’:
23-2.c:12: warning: #warning The macro Product is NOT defined!
~/will$ 
~/will$ ./a.out
1. Query Information.
2. Record Information.
3. Delete Information.

條件編譯：

~/will$ gcc -DPRODUCT=3 23-2.c
~/will$ ./a.out
This is a high level product!
1. Query Information.
2. Record Information.
3. Delete Information.
4. High Level Query.
5. Mannul Service.
6. Exit.

#line預處理器指示字

    #line用於強制指定新的行號和編譯文件名，並對源程序的代碼重新編號。
用法：

		#line number filename
		filename 可省略

#line編譯指示字的本質是重定義 __FILE__ 和 __LINE__

行號從重新定義的文件名起始計算。（從最靠近主函數的地方開始計算）

#include <stdio.h>

// The code section is written by A.
// Begin
#line 1 "a.c"

// End

// The code section is written by B.
// Begin
#line 1 "b.c"

// End

// The code section is written by Delphi.
// Begin
#line 1 "willwilling's.c"


int main()
{
    printf("%s : %d\n", __FILE__, __LINE__);
    
    printf("%s : %d\n", __FILE__, __LINE__);
    
    return 0;
}

// End

編譯運行：

~/will$ gcc 23-3.c
~/will$ ./a.out
willwilling's.c : 5
willwilling's.c : 7

小結： 

                 #error用於自定義一條編譯錯誤

#warning用於自定義一條編譯警告信息

#error和#warning常用於條件編譯的情形

#line用於強制指定新的行號和編譯文件名。