how to use a c++ function in c code, also how to link two languages?

Could anybody help me in solving the problem of using c++ function/class in C code.
Given below is my code

Code:

---

//C code
#include <stdio.h>
//#include <code_cpp.cc>
extern int value(int, char**);
int main(int argc, char* argv[])
{
  //main(int);
  //int c;
  return value(argc, argv);
  int a = 1;
  printf("value of a is %d \n", a);
  return 0;
}

---

Code:

---

//C++ code
#include<iostream>
#ifdef __cplusplus
extern "C" int value()
{
#endif
  int b = 2;
  return b;
#ifdef __cplusplus
 } // extern "C"
 #endif

extern "C" int main ()
{
  //std::cout << "Hello, world!\n";
  int main_b;
  main_b = value();
  std::cout<<"value of main_b is "<<main_b<<std::endl;
  return main_b;
}

---

On compiling c code I got the following error

Code:

---

gcc -Wall code_c.c -o code_c
/tmp/cctXTHYe.o: In function `main':
code_c.c:(.text+0x17): undefined reference to `value'
collect2: ld returned 1 exit status

---

I think there is some linking problem.

Thanks in advance!

Look at your preprocessor commands. You exclude the whole function. Instead, you want to exclude the extern only.

Code:

---

#ifdef __cpluplus
extern "C"
{
#endif

int value()
{
    // ...
}

#ifdef __cplusplus
}
#endif

---

There are two sets of brackets here: those that include the extern "C" and those that include the function body. Equivalently, you could have conditionally excluded only the symbols extern "C" .

Code:

---

#ifdef __cplusplus
extern "C"
#endif

int value()
{
    // ...
}

---

How to mix C and C++, C++ FAQ

in your C++ file you're declaring it to take no parameters

Quote:

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Originally Posted by vermaric

Code:

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extern "C" int value()

---

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and then in your C code, you're declaring it to take two parameters. that is also a problem. it is likely to cause all sorts of undefined behavior.

Quote:

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Originally Posted by vermaric

Code:

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extern int value(int, char**);

---

---

Actually, in C if you declare a function prototype's argument list as (), this means "I don't know and I don't care what the argument list is", which is different from saying (void), which means "This function takes no parameters". And in C's case the code will still compile and link either way, since name-mangling doesn't occur (the symbol name will be the same regardless of which arguments the function takes).

So the worst that will happen is you get undefined arguments in the function. Which is pretty bad, really. But you probably won't get errors about it, just warnings, which is another reason to pay attention to warnings.

Quote:

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Originally Posted by dwks

Actually, in C if you declare a function prototype's argument list as (), this means "I don't know and I don't care what the argument list is", which is different from saying (void), which means "This function takes no parameters". And in C's case the code will still compile and link either way, since name-mangling doesn't occur (the symbol name will be the same regardless of which arguments the function takes).

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Depending on the compiler and the call options (how many input parameters are register based), you get name mangling in C as well (usually the name is suffixed by @xx, where xx is the number of parameter bytes pushed onto the stack). I sometimes combine assembly and C, and to get the names, I have the compiler generate assembly code for the C code to get the function names.