cgo：体系结构x86_64的未定义符号

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 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