c++ 的链接指示器 extern "C"

1. c++ 使用c的函数

c++ 标准库使用 c标准库，比如

cstdio   
/** @file include/cstdio
 *  This is a Standard C++ Library file.  You should @c #include this file
 *  in your programs, rather than any of the "*.h" implementation files.
 *
 *  This is the C++ version of the Standard C Library header @c stdio.h,
 *  and its contents are (mostly) the same as that header, but are all
 *  contained in the namespace @c std (except for names which are defined
 *  as macros in C).
 */


  extern "C" int
  (snprintf)(char * restrict, size_t, const char * restrict, ...);

再来一个简单例子

-- math.h

#ifndef __MATH_H__
#define __MATH_H__

#ifdef __cplusplus
extern "C" {
#endif

int func(int,int);

#ifdef __cplusplus
}
#endif

#endif

-- math.c

#include "math.h"
int func(int a,int b)
{
    return a+b;
}

-- 编译成静态库：
-- gcc -c math.c
-- ar -r librmath.a *.o

-- math.cpp

#include "math.h"
#include <iostream>
using namespace std;
int main()
{
    int a= 2,b = 3;
    int c = func(a,b);  // 直接调用c接口
    cout << c << endl;
}

-- 编译成可执行程序：
-- g++ math.cpp -L./ -lrmath

-- ./a.out

如果没有加上extern "C"，则编译时报链接错误：
math.cpp:(.text+0x21): undefined reference to `func(int, int)'
collect2: ld returned 1 exit status

2. 导出c++函数到c语言

-- math.h

class sample {
    public:
        int func(int a, int b);
};

-- math.cpp

#include "math.h"
int sample::func(int a, int b)
{
    return a + b;
}

-- g++ -fpic -shared -g -o librmath.so math.cpp -I ./

写c代码包装一下

-- mymath.h

#ifdef __cplusplus
extern "C" {
#endif

int myfunc(int, int);

#ifdef __cplusplus
}
#endif

-- mymath.c

#include "mymath.h"

#include "math.h"

int myfunc(int a, int b)
{
    sample s;
    return s.func(a, b);
}

-- g++ -fpic -shared -g -o libmymath.so mymath.cpp -lrmath -I ./ -L ./

测试一下：
-- main.c

#include <stdio.h>
#include "mymath.h"
int main()
{
    printf("%d\n", myfunc(1, 2));
    return 0;
}

-- gcc main.c -L./ -I ./ -lrmath -lmymath -Wl,-rpath=./

-- ./a.out

如果想要编译成静态库：

g++ -c math.cpp

ar -r librmath.a math.o

g++ -c mymath.cpp

ar -r libmymath.a mymath.o

gcc main.c libmymath.a librmath.a -lstdc++

注意两点

1. -lstdc++

undefined reference to `__gxx_personality_v0'

因为：

Note that programs using C++ object files must always be linked with g++, in order to supply the appropriate C++ libraries. Attempting to link a C++ object file with the C compiler gcc will cause "undefined reference" errors for C++ standard library functions

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

大概意思是gcc根据后缀名去链接标准库，上面例子gcc没有检测到c++，所以默认不去链接c++基础库。

2. main.c libmymath.a librmath.a 这三个的顺序不能动，因为:

When the GNU linker sees a library, it discards all symbols that it doesn't need. In this case, your library appears before your .cpp file, so the library is being discarded before the .cpp file is compiled.

所以推荐动态库的方式。

c++ 的链接指示器 extern "C"

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