Can Cgo call C function declared in another directory?

Question

It seems that I can't use Cgo to call a C function declared in another directory rather than current Go package.

Codes of all files :

// TestGoCallOC.go
package main
/*
#include "test.h"
#cgo CFLAGS: -x objective-c
#cgo LDFLAGS: -framework Foundation
*/
import "C"
import (
    "fmt"
)
func main()  {
    fmt.Println(C.fortytwo())
}

// test.h
#include <stdio.h>
#include <stdlib.h>
int fortytwo();

// test.m
#include "test.h"
int fortytwo() {
    return 42;
}

If I put all the files in one directory:

|--src
   |--TestGoCallOC
      |--TestGoCallOC.go
      |--test.h
      |--test.m

and run the Go main function, these two C functions could be called.

But if I put C files (actually they are Objective-C files) in another direcory:

|--src
   |--TestGoCallOC
      |--TestGoCallOC.go
   |--SomeOCCodes
      |--test.h
      |--test.m

, change the file path of #include "test.h" in preamble to it absolute path, and run the Go main function, these two C functions could not be called.

Error message is

Undefined symbols for architecture x86_64: "_fortytwo", referenced from: __cgo_b3674ecbf56b_Cfunc_fortytwo in TestGoCallOC.cgo2.o (maybe you meant: __cgo_b3674ecbf56b_Cfunc_fortytwo) ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) exit status 2

Did I do something wrong? Or is Cgo not capable of doing this?

Answer 1

according to https://golang.org/cmd/cgo/
All the cgo CPPFLAGS and CFLAGS directives in a package are concatenated and used to compile C files in that package. All the CPPFLAGS and CXXFLAGS directives in a package are concatenated and used to compile C++ files in that package. All the LDFLAGS directives in any package in the program are concatenated and used at link time. All the pkg-config directives are concatenated and sent to pkg-config simultaneously to add to each appropriate set of command-line flags.

Go package boundary is src folder so you may put all c files in same folder/ or use include C file(not h file) workaround like this:

// main.go
package main

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

func main() {
    r := C.Add(10, 20)
    fmt.Println(r)
}

and c file in ctest dir:

//test.c

int Add(int a, int b){
    return a+b;
}

this works fine.