Call a C++ function from C

Question

A very simple question. I was trying to call a c++ function from a c file. I always seem to get "undefined reference to the function" error.

C file: test.c

#include "link.h"      
#include <stdio.h>


int main(){   
    print();
    return 0;
}

C++ file: print.cpp

#include <iostream>
#include "link.h"

extern "C"
void print(){
   std::cout<<"hello from cpp";
}

Header file: link.h

 #ifndef HEADER_FILE
 #define HEADER_FILE

 #ifdef __cplusplus
     extern "C" {
 #endif
         void print();
 #ifdef __cplusplus
     }
 #endif

 #endif

I don't know what's wrong with the above code. Also, how do I compile the above in terminal? I'm assuming I need to use the gcc compiler (gcc -o test test.c link.h)

Any inputs will be highly appreciated. Thanks,

You don't seem to link print.cpp. Which can never be compiled as C. — Lundin, Nov 07 '22 at 11:25
You must provide the file where the function is defined to your compiler/linker as well. — Gerhardh, Nov 07 '22 at 11:25
fwiw, you add an `extern "C"` in the header depending on `__cplusplus`, but you do it unconditionally in the source file. — andreee, Nov 07 '22 at 11:27
`gcc ... link.h` doesn't do what you think it does. You should not try to compile .h files. — n. m. could be an AI, Nov 07 '22 at 11:28
You are not including your C++ file in the compilation command `g++ -o test test.c print.cpp` should do the job (note, g++, because you are compiling C++ as well). — andreee, Nov 07 '22 at 11:28
@andreee that's perfectly fine. The source file can only be compiled as C++. There is no condition to check. — n. m. could be an AI, Nov 07 '22 at 11:29
`g++ test.c` will compile `test.c` as C++ code, which is probably not what OP wants. — n. m. could be an AI, Nov 07 '22 at 11:30
@n.m. but will it automatically set the correct linking flags to include the standard library? IIRC, that's not the case with the plain gcc driver, but things may have changed... — andreee, Nov 07 '22 at 11:30
@andreee If it does one thing wrong, it doesn't matter if it does other 99 things right. If you want to compile a C file as C and a C++ file as C++, you use two different commands that run two different compilers. — n. m. could be an AI, Nov 07 '22 at 11:32
I'd say the bulletproof solution is `g++ -c print.cpp; gcc -c test.c; g++ -o test print.o test.o` — andreee, Nov 07 '22 at 11:32
@andreee And now please write an actual answer. This is not a forum. ;-) — the busybee, Nov 07 '22 at 11:37

Call a C++ function from C

0 Answers0