主題: 在 C++ 中引用 C 語言編成的函式

[劍客]

使用外部函式時，使用 extern 宣告。但是如果所呼叫的函式與呼叫者本身的語言不同時，可以指定所呼叫的函式的編譯語言。

範例如下；使用時打 make ，編譯成扑|產生一個 sum 的執行檔。大家可以換成註解的宣告再make 一次看看有什麼差別。

// filename: main.c
#include
extern "C" int sum(int a, int b);
/*
 * 錯誤的宣告
 *
 * extern int sum(int a, int b);
*/
int main(void){
        cout << "Sum is : " << sum(3,5) << endl;
        return 0;
}

// filename sum.c
#include
int a;
int b;
int sum(int a, int b){
        return a+b;
}

# filename Makefile
all:
        cc -c sum.c
        c++ -o sum main.c sum.o

[bluerain]

1. C ++ invoke C :

// cfun.c
#include <stdio.h>

void cfunc(num1, num2, res)
int num1, num2, *res;
{    
   printf("func: a = %d b = %d ptr c = %x\n",num1,num2,res);
    *res=num1/num2;    
   printf("func: res = %d\n",*res);
}

//cppmain.cpp
extern "C" void cfunc(int n, int m, int *p);

#include <iostream.h>
void main()
{   int a,b,c;  
   a=8;  
   b=2;  
   cout << "main: a = "<<a<<" b = "<<b<<" ptr c = "<<&c<< endl;
   cfunc(a,b,&c);  
   cout << "main: res = "<<c<<endl;
}

2. C invoke C++:

// cmain.c
extern void cpfunc(int a, int b, int *c);

#include <stdio.h>
void main()
{   int a,b,c;  
   a=8;  
   b=2;  
   printf("main: a = %d b = %d ptr c = %x\n",a,b,&c);  
   cpfunc(a,b,&c);  
   printf("main: res = %d\n",c);
}

//cppfunc.cpp
#include <iostream.h>

extern "C" void cpfunc(int num1,int num2,int *res)
{  
   cout << "func: a = "<<num1<<" b = "<<num2<<" ptr c ="<<res<<endl;  
   *res=num1/num2;  
   cout << "func: res = "<<res<<endl;
}

[bluerain]

1. C++ invoke C:

// cfunc.c
#include <stdio.h>

void cfunc(alarm,res)
int alarm;
char **res;
{    
   int i;
   printf("func: alarm = %d ptr c = %x\n",alarm,&res);
    i = (alarm==0)?1:0;
   
   switch(i){
       case 1  : *res = "true";break;
       case 0  :
      default : *res = "false";break;
   }

   printf("func: res = %s\n",*res);
}

// cppmain.cpp
extern "C" void cfunc(int x, char **y);

#include <string.h>
#include <iostream.h>

void main()
{  
   char *str="";
   int a;
   char *c="test";
   cout<<"Enter 0/1 : ";
   cin>>a;
   
   switch(a)
   {
      case 1  : str = "true";//strcpy(str,"true");
               break;
      case 0  :
      default : str = "false";//strcpy(str,"false");
               break;
   }

   cout << "main: alarm = "<<str<<" ptr c = "<<&c<< endl;
   cfunc(a,&c);  
   cout << "main: res = "<<c<<endl;
}

2. C invoke C++ :

//cppfunc.cpp
#include <iostream.h>

extern "C" void cpfunc(int alarm,char** res)
{    
   int i;
   cout<<"func: alarm = "<< alarm <<" ptr c = "<<&res<<endl;
   
   i = (alarm==0)?1:0;
   
   switch(i){
       case 1  : *res = "true";break;
       case 0  :
      default : *res = "false";break;
   }

   cout<<"func: res = "<<*res<<endl;
}

// cmain.c
extern void cpfunc(int a, char** c);

#include <stdio.h>
void main()
{
   char *str="";
   int a;
   char *c="test";
   printf("Enter 0/1 : ");
   scanf("%d",&a);
   
   switch(a)
   {
      case 1  : str = "true";//strcpy(str,"true");
               break;
      case 0  :
      default : str = "false";//strcpy(str,"false");
               break;
   }

   printf("main: alarm = %s ptr c = %x\n",str,&c);  
   cpfunc(a,&c);  
   printf("main: res = %s\n",c);
}