1. C 调用C++
// C++头文件，example.h
extern "C" void Print(int i);
extern "C" int g_num;
// C头文件，example_c.h
void Print_C(int i);
extern int g_num_c;
//C++文件，example.cpp
#include <iostream>
#include "example.h"
void Print(int i)
{
std::cout<<"This is a function defined in C++,variable defined in a c++ file:"<<i<<std::endl;
}
int g_num = 1;
//C文件，example_c.c
#include <stdio.h>
void Print_C(int i)
{
printf("This is a function defined in C,variable defined in a c file:%d\n",i);
}
int g_num_c = 0;
extern void Print(int i);
extern int g_num;
void main()
{
printf("C call C++\n");
Print(g_num);
printf("C call C\n");
Print_C(g_num_c);
}
结果：
C call C++
This is a function defined in C++,variable defined in a c++ file:1
C call C
This is a function defined in C,variable defined in a c file:0
C 调用C++的函数或变量，在C++的头文件声明为extern "C" ，C调用的时候只使用extern 声明。
查看目标文件：
D:\workspace\HEVC\C++vsC\C_C++_call>dumpbin /RELOCATIONS example_c.obj
Microsoft (R) COFF Binary File Dumper Version 6.00.8168
Copyright (C) Microsoft Corp 1992-1998. All rights reserved.
Dump of file example_c.obj
File Type: COFF OBJECT
RELOCATIONS #2
Symbol Symbol
Offset Type Applied To Index Name
-------- ---------------- ----------------- -------- ------
000000AA SECREL 00000000 C _Print_C
000000AE SECTION 0000 C _Print_C
000000EB SECREL 00000000 17 _main
000000EF SECTION 0000 17 _main
00000103 SECREL 00000000 9 _g_num_c
00000107 SECTION 0000 9 _g_num_c
00000119 SECREL 00000000 1B _g_num
0000011D SECTION 0000 1B _g_num
symbol为函数变量名前加_ .
dumpbin /RELOCATIONS example.obj>2.txt 可以看到C++中使用了extern "C"声明，目标文件中的symbol也是_g_num ，_Print.
如果注释掉example.cpp 中#include "example.h"
example_c.obj : error LNK2001: unresolved external symbol _Print
example_c.obj : error LNK2001: unresolved external symbol _g_num
出现链接错误，dumpbin /SYMBOLS example.obj>2.txt，C++的链接规则生成的symbol为 ?g_num@@3HA (int g_num)，?Print@@YAXH@Z (void __cdecl Print(int)).
可见，extern "C"告诉了C++的编译器，使用C的链接规则生成和寻找目标文件中的symbol名称。
2. C++ 调用C
//C++文件，example.cpp
#include <iostream>
//#include "example.h"
void Print(int i)
{
std::cout<<"This is a function defined in C++,variable defined in a c++ file:"<<i<<std::endl;
}
int g_num = 1;
extern "C"
{
#include "example_c.h"
}
void main()
{
std::cout<<"C++ call C"<<std::endl;
Print_C(g_num_c);
std::cout<<"C++ call C++"<<std::endl;
Print(g_num);
}
结果：
C++ call C
This is a function defined in C,variable defined in a c file:0
C++ call C++
This is a function defined in C++,variable defined in a c++ file:1
查看目标文件：
dumpbin /SYMBOLS example.obj>2.txt ， symbol为：_Print_C ， _g_num_c ；?Print@@YAXH@Z (void __cdecl Print(int))，?g_num@@3HA. (int g_num)
总之 extern "C" { } ，声明用于C++中，告诉编译器对{ }中声明的函数或变量使用C的方式生成（或寻找）目标符号。
3. C++ 调用C中的内联函数
C中的函数使用__inline声明为内联函数时，不会链接生成目标符号。
example_c.c中的函数改为 __inline void Print_C(int i) ，在example_c.obj找不到Print_C的符号。
可以使用__declspec( dllexport ) __inline void Print_C(int i) 这样的声明，就会生成目标符号 External | _Print_C 。
参考:msdn,Using extern to Specify Linkage ,http://msdn.microsoft.com/zh-cn/library/0603949d.aspx