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Happy coding

extern "C" 的用意

Ted posted @ 2008年8月12日 18:17 in 未分类 with tags extern c , 2701 阅读

前些天,编程序是用到了很久以前写的C程序,想把里面的函数利用起来,连接发现出现了找不到具体函数的错误:

以下是假设旧的C程序库

C的头文件

/*-----------c.h--------------*/
#ifndef _C_H_
#define _C_H_
extern int add(int x, int y);
#endif

C的源文件

/*-----------c.c--------------*/
int
add(int x, int y){
return
x+y;
}

C++的调用

/*-----------cpp.cpp--------------*/
#include "c.h"
void main()
{
add(1, 0);
}

这样编译会产生错误cpp.obj : error LNK2001: unresolved external symbol "int __cdecl add(int,int)" (?add@@YAHHH@Z),原因是找不到add的目标模块

这才令我想起C++重载的函数命名方式和C函数的命名方式,让我们回顾一下:C中函数编译后命名会在函数名前加以"_",比如add函数编译成obj文件时的实际命名为_add,而c++命名则不同,为了实现函数重载同样的函数名add因参数的不同会被编译成不同的名字

例如

int add(int , int)==>add@@YAHHH@Z,

float add(float , float )==>add@@YAMMM@Z,

以上是VC6的命名方式,不同的编译器会不同,总之不同的参数同样的函数名将编译成不同目标名,以便于函数重载是调用具体的函数。

编译cpp.cpp中编译器在cpp文件中发现add(1, 0);的调用而函数声明为extern int add(int x, int y);编译器就决定去找add@@YAHHH@Z,可惜他找不到,因为C的源文件把extern int add(int x, int y);编译成_add了;

为了解决这个问题C++采用了extern "C",这就是我们的主题,想要利用以前的C程序库,那么你就要学会它,我们可以看以下标准头文件你会发现,很多头文件都有以下的结构

#ifndef __H
#define __H
#ifdef __cplusplus
extern "C" {
#endif

extern
int f1(int, int);
extern
int f2(int, int);
extern
int f3(int, int);


#ifdef __cplusplus
}
#endif

#endif /*__H*/

如果我们仿制该头文件可以得到

#ifndef _C_H_
#define _C_H_
#ifdef __cplusplus
extern "C" {
#endif

extern
int add(int, int);

#ifdef __cplusplus
}
#endif

#endif /* _C_H_ */

这样编译

/*-----------c.c--------------*/
int
add(int x, int y){
return
x+y;
}

这时源文件为*.c,__cplusplus没有被定义,extern "C" {}这时没有生效对于C他看到只是extern int add(int, int);
add函数编译成_add(int, int);

而编译c++源文件

/*-----------cpp.cpp--------------*/
#include "c.h"
void main()
{

add(1, 0);
}

这时源文件为*.cpp,__cplusplus被定义,对于C++他看到的是extern "C" {extern int add(int, int);}编译器就会知道 add(1, 0);调用的C风格的函数,就会知道去c.obj中找_add(int, int)而不是add@@YAHHH@Z

这也就为什么DLL中常看见extern "C" {},windows是采用C语言编制他首先要考虑到C可以正确调用这些DLL,而用户可能会使用C++而extern "C" {}就会发生作用
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