本文将以 go 标准库中的 sync/pool 为例探讨导出 unexported 项的方法

通过 go:linkname 来导出私有全局变量

package main

import _ "unsafe"

//go:linkname poolRaceHash sync.poolRaceHash
var poolRaceHash [128]uint64

func main() {
    // main.poolRaceHash is an alias of sync.poolRaceHash
    // -> unexported item sync.poolRaceHash is stolen
    var _ = poolRaceHash
}

可通过裸指针直接操作内存来导出私有 field

package main

import (
    "sync"
    "unsafe"
)

type Pool struct {
    noCopy noCopy

    local     unsafe.Pointer
    localSize uintptr

    New func() interface{}
}

type noCopy struct{}

func main() {
    // literally construct a `sync.Pool`
    var externPool = sync.Pool{
        New: func() interface{} {
            return nil
        },
    }
    var pool = (*Pool)(unsafe.Pointer(&externPool))
    // unexported item `sync.Pool.local` is stolen
    _ = pool.local
}

package main

import (
    "sync"
    "unsafe"
)

type Pool struct {
    noCopy noCopy

    local     unsafe.Pointer
    localSize uintptr

    New func() interface{}
}

type noCopy struct{}

type poolLocal struct {
    private interface{}
    shared  []interface{}
    sync.Mutex
    pad [128]byte
}

//go:linkname pinSlow sync.(*Pool).pinSlow
func pinSlow(p *sync.Pool) *poolLocal

//go:linkname (*Pool).pinSlow sync.(*Pool).pinSlow
func (p *Pool) pinSlow() *poolLocal

//go:linkname poolRaceAddr sync.poolRaceAddr
func poolRaceAddr(x interface{}) unsafe.Pointer

func main() {
    // literally construct a `sync.Pool`
    var externPool = sync.Pool{
        New: func() interface{} {
            return nil
        },
    }
    var pool = (*Pool)(unsafe.Pointer(&externPool))

    // main.pinSlow is an alias of sync.(*Pool).pinSlow
    // -> unexported item sync.(*Pool).pinSlow is stolen
    var _ = pinSlow(&externPool)

    // main.(*Pool).pinSlow is an alias of sync.(*Pool).pinSlow
    // -> unexported item sync.(*Pool).pinSlow is stolen
    var _ = (*Pool).pinSlow(pool)

    // main.poolRaceAddr is an alias of sync.poolRaceAddr
    // -> unexported item sync.poolRaceAddr is stolen
    var _ = poolRaceAddr(nil)
}

在项目中的新建一个空白的、任意名称的汇编代码文件（比如 asm.s）以帮助编译器完成函数完整性检查

go_export
├── asm.s
└── main.go

$ go build

go-forceexport 提供了这种便利

• 当使用 go:linkname 导出非标准库中的私有项时，须在当前包中显式地 import 来源包

参考资料：https://sitano.github.io/2016/04/28/golang-private/