cgo : Undefined symbols for architecture x86_64

Question

I would like to call the go func from the C function space, but the program throws the build error.

example.go

package main

/*
#include "test.c"
*/
import "C"
import "fmt"

func Example() {
    fmt.Println("this is go")
    fmt.Println(C.GoString(C.myprint(C.CString("go!!"))))
}

// export receiveC              (remove the extra space between // and export)
func receiveC(msg *C.char) {
    fmt.Println(C.GoString(msg))
}

func main() {
    Example()
}

test.c

#include <stdio.h>

extern void receiveC(char *msg);

char* myprint(char *msg) {
    receiveC(msg);  // calling the exported go function
    return msg;     
    }

When I execute the command to run/build (go build or go run example.go or go build example.go) the program, it throws the error:

# github.com/subh007/goodl/cgo
Undefined symbols for architecture x86_64:
  "_receiveC", referenced from:
      _myprint in example.cgo2.o
      __cgo_6037ec60b2ba_Cfunc_myprint in example.cgo2.o
      _myprint in test.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I am following the cgo slides to writing the program. Please let me know for any mistakes here.

Edit1: I am using OS-X 10.9 OS.

Edit2: I have one extra space between // export, there should be no space between // and export. But now I am getting the following error while building :

# github.com/subh007/goodl/cgo
duplicate symbol _myprint in:
    $WORK/github.com/subh007/goodl/cgo/_obj/_cgo_export.o
    $WORK/github.com/subh007/goodl/cgo/_obj/example.cgo2.o
duplicate symbol _receiver_go in:
    $WORK/github.com/subh007/goodl/cgo/_obj/_cgo_export.o
    $WORK/github.com/subh007/goodl/cgo/_obj/example.cgo2.o
duplicate symbol _myprint in:
    $WORK/github.com/subh007/goodl/cgo/_obj/_cgo_export.o
    $WORK/github.com/subh007/goodl/cgo/_obj/test.o
duplicate symbol _receiver_go in:
    $WORK/github.com/subh007/goodl/cgo/_obj/_cgo_export.o
    $WORK/github.com/subh007/goodl/cgo/_obj/test.o
ld: 4 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

Duplicate symbols are generated because I have included the test.c directly to the go file. So the symbols are included twice.

I think, the correct way of writing this code is (please comment if I am wrong) to:

1. Define the header file (test.h):

    #ifndef TEST_H_
    #define TEST_H_
   
    char* myprint(char *msg);
   
    #endif
   

2. Define the implementation file (test.c):

     #include <stdio.h>
     #include "test.h"
   
     extern void receiveC(char *msg);
   
     char* myprint(char *msg) {
         receiveC(msg);
         return msg;
      }
   

3. Include the .h file to the go file (example.go) :

     package main
     /*
     #include "test.h"
     */
     import "C"
     import "fmt"
   
     func Example() {
          fmt.Println("this is go")
          fmt.Println(C.GoString(C.myprint(C.CString("go!!"))))
      }
   
      // make sure that there should be no space between the `//` and `export`
      //export receiveC
      func receiveC(msg *C.char) {
            fmt.Println(C.GoString(msg))
      }
   
      func main() {
           Example()
      }
   

4. Build the program :

     go build
   

5. run the generated executable (the executable file generate with the cgo name, need some investigation to find the reason).

     $./cgo