Using Go on existing C project

Question

I have a program entirely written in C that uses multiple object (.o) files in it. These files are all packed inside an archive file (.a) which, in turn, is used at compile-time of the program's main (.c) file.

I want to write a new file for this project in Go. My idea is to write this .go file and then create an object (.o) file from it. Afterwards, I want to put this object file inside the already mentioned archive (.a) file.

This basically means that I want to call Go functions from a C program. I've read this question, and while it showed me that what I want is possible via GCCGO, it's not 100% clear as to how to do it.

Even with the most basic of tests, I get errors during the linking phase. More specifically, here's one of such basic example:

---

printString.go

package main

import
(
    "fmt"
)

func PrintString(buff string) int {
    fmt.Printf(buff)
    return 1
}

c_caller.c

#define _GNU_SOURCE
#include <stdio.h>

extern int PrintString(char*) __asm__ ("print.main.PrintString");

int main() {
    char *string_to_pass= NULL;
    asprintf(&string_to_pass, "This is a test.");

    int result= PrintString(string_to_pass);
    if(result) {printf("Everything went as expected!\n");}
    else       {printf("Uh oh, something went wrong!\n");}

    return result;
}

Compiling

In order to compile the Go file, I used this command:

gccgo -c printString.go -o printString.o -fgo-prefix=print -Wall -Werror -march=native

In order to compile the entire thing, I used this command:

gccgo -o main c_caller.c printString.o -Wall -Werror -march=native

The return message I'm getting is:

/usr/lib64/libgo.so.4.0.0: undefined reference to `main.main'
/usr/lib64/libgo.so.4.0.0: undefined reference to `__go_init_main'
collect2: error: ld returned 1 exit status

Which means that GCCGO's expecting a main function in the Go file instead of the C one.

Using the --static-libgo, -static and -Wl,-R,/path/to/libgo.so's_folder options on the second command yield a different result:

/usr/bin/ld: cannot find -lgo
collect2: error: ld returned 1 exit status

Which makes no sense, since I have the LD_LIBRARY_PATH environment variable properly pointing to libgo.so's folder.

---

I realize that I'm probably doing something wrong here, but I just can't see what that is. There's next to no examples of GCCGO and its interaction with C out there, and the only reference I could find was this page, which I personally feel like it's not enough.

I ask kindly for some advice on this matter and thank you for your time. :)

Answer 1

This may not be what you want, but in Go 1.5, that's coming this August, you'll be able to build C-compatible libraries with the go tool. So with this in your _main.c

#include <stdio.h>

int main()
{
    char *string_to_pass = NULL;
    if (asprintf(&string_to_pass, "This is a test.") < 0) {
        printf("asprintf fail");
        return -1;
    }

    PrintString(string_to_pass);
    return 0;
}

and this in your main.go

package main

import "C"
import "fmt"

//export PrintString
func PrintString(cs *C.char) {
    s := C.GoString(cs)
    fmt.Println(s)
}

func main() {}

You can do, for static library:

go build -buildmode c-archive -o mygopkg.a
gcc -o main _main.c mygopkg.a -lpthread

For shared library:

go build -buildmode c-shared -o mygopkg.so
LD_RUN_PATH=$(pwd) gcc -o main _main.c mygopkg.so -lpthread

(LD_RUN_PATH is here to make the linker look for the shared library in the same directory you're building.)

See the Go execution modes design document for more info.