golang cgo无法通过构建模式c共享导出变量

Question

I am trying to develop in the cgo a plug-in of sudo.

https://www.sudo.ws/man/1.8.15/sudo_plugin.man.html

export the struct to the global scope of policy_plugin.

A policy plugin must declare and populate a policy_plugin struct in the global scope.

Do you have an explanation of what that means?

export_test.go

package main

/*
#include "sudo_plugin.h"
#include <stddef.h>
*/
import "C"

func main() {
}

// don't worked
//export policy
var policy = &C.struct_policy_plugin{
    C.SUDO_POLICY_PLUGIN,
    C.SUDO_API_VERSION,
    nil,
    nil,
    nil,
    nil,
    nil,
    nil,
    nil,
    nil,
    nil,
    nil,
}

Answer 1

It appears buildmode c-shared only exports function entries, not variables. While the design doc does not state this explicitly, it does not talk about variables explicitly either.

The cmd/cgo manual also does not mention variables explicitly but seems to imply that the special //export ... comments apply to functions only.

What you could supposedly do about this is declare your external variable on the C side—after defining and expoting the necessary Go functions, like this:

"callbacks.go":

package main

import "C"

//export real_fn
func real_fn(x C.int) C.int {
    return 42
}

"main.go":

package main

/*
typedef struct {
    int x;
    int (*fn) (int x);
} foo;

extern int real_fn(int);

foo xyzzy = {
    0,
    real_fn,
};
*/
import "C"

func main() {
}

Now after running go build -buildmode=c-shared you can explore the generated library and see the xyzzy symbol available there:

cshared% nm -g cshared | grep -E 'xyzzy|real_fn'
0000000000061330 T _cgoexp_4f8dd74b8333_real_fn
00000000000b42c0 T real_fn
00000000003274d0 D xyzzy

A note: if you're about using callbacks you have to put them into a separate file (or files)—this appears to be a quirk of cgo.