一个匿名函数引用它的上下文对象,把这种状态称为闭包。闭包由两部分组成：
- 函数
- 环境变量
当一个函数有局部变量x,当这个函数返回一个闭包,闭包通过指针引用环境变量,使环境变量生命周期延长,此时变量x会逃逸到heap上。只有这样调用闭包函数时,才能访问到x的值。
package main
func test(x int) func() {
println("test.x :", &x)
return func() {
println("closure.x :", &x, x)
}
}
func main() {
f := test(100)
f()
}
查看源代码编译之后的elf文件符号表、gdb调试及调用堆栈分析等手段查看Go 闭包是如何实现的。
- 编译 & 查看符号表
$> go build -gcflags "-l -m -N" -o test main.go
$> nm test | grep "main\."
000000000104e2f0 t main.init
00000000010d5a82 s main.initdone.
000000000104e220 t main.main
000000000104e140 t main.test
000000000104e270 t main.test.func1
0000000001045b30 t runtime.main.func1
0000000001045b80 t runtime.main.func2
- gdb 调试
Dump of assembler code for function main.main:
0x000000000104e220 <+0>: mov rcx,QWORD PTR gs:0x30
0x000000000104e229 <+9>: cmp rsp,QWORD PTR [rcx+0x10]
0x000000000104e22d <+13>: jbe 0x104e263 <main.main+67>
0x000000000104e22f <+15>: sub rsp,0x20
0x000000000104e233 <+19>: mov QWORD PTR [rsp+0x18],rbp
0x000000000104e238 <+24>: lea rbp,[rsp+0x18]
; [rsp]通过寄存器传参
=> 0x000000000104e23d <+29>: mov QWORD PTR [rsp],0x64
0x000000000104e245 <+37>: call 0x104e140 <main.test>
; rdx是test函数返回值
0x000000000104e24a <+42>: mov rdx,QWORD PTR [rsp+0x8]
0x000000000104e24f <+47>: mov QWORD PTR [rsp+0x10],rdx
0x000000000104e254 <+52>: mov rax,QWORD PTR [rdx]
0x000000000104e257 <+55>: call rax
0x000000000104e259 <+57>: mov rbp,QWORD PTR [rsp+0x18]
0x000000000104e25e <+62>: add rsp,0x20
0x000000000104e262 <+66>: ret
0x000000000104e263 <+67>: call 0x1046ae0 <runtime.morestack_noctxt>
0x000000000104e268 <+72>: jmp 0x104e220 <main.main>
; rdx是一个复合类型
; rdx[0]: main.test.func1函数,test函数内返回函数的内存地址
; rdx[1]: test函数环境变量
gdb) p/x $rdx
$4 = 0xc00000e020
(gdb) x/2xg 0xc00000e020
0xc00000e020: 0x000000000104e270 0x000000c0000160d0
(gdb) x/xg 0x000000000104e270
0x104e270 <main.test.func1>: 0x000030250c8b4865
(gdb) x/xg 0x000000c0000160d0
0xc0000160d0: 0x0000000000000064
通过分析,可以得知test函数返回的是一个复合结构体,由两个元素组成.第一个保存闭包函数地址,第二个是环境变量地址(环境变量会逃逸到heap上)
// 逃逸分析
$> go build -gcflags "-l -m -N" -o test main.go
# command-line-arguments
./closure.go:6:12: func literal escapes to heap
./closure.go:6:12: func literal escapes to heap
# 环境变量逃逸到heap
./closure.go:7:18: &x escapes to heap
./closure.go:3:11: moved to heap: x
./closure.go:4:13: test &x does not escape
./closure.go:7:17: test.func1 &x does not escape
当函数返回多个闭包函数时,如果闭包之间都引用了相同的环境变量,此时会有数据竞争的问题,在闭包调用时需要用锁进行处理处理。
package main
func test(x int) (func(), func()) {
w := func() {
for {
x++
}
}
r := func() {
for {
_ = x
}
}
return w, r
}
func main() {
w, r := test(1000)
go w()
go r()
}
- 数据竞争检查
$> go run -race closure.go
==================
WARNING: DATA RACE
Read at 0x00c00007e000 by goroutine 6:
main.test.func2()
/Users/David/data/go/go.test/src/Demo/closure.go:13 +0x46
Previous write at 0x00c00007e000 by goroutine 5:
main.test.func1()
/Users/David/data/go/go.test/src/Demo/closure.go:7 +0x5c
Goroutine 6 (running) created at:
main.main()
/Users/David/data/go/go.test/src/Demo/closure.go:24 +0x63
Goroutine 5 (running) created at:
main.main()
/Users/David/data/go/go.test/src/Demo/closure.go:23 +0x4d
==================
Found 1 data race(s)
exit status 66