loopvar 改动不同版本的影响-并发
看一个关于并发的例子
package main
import (
"fmt"
"runtime"
"sync"
)
func main() {
fmt.Println("golang version:", runtime.Version())
concurrencyDemo()
}
func concurrencyDemo() {
isGold := func(num uint64) bool {
return num%65536 == 0
}
var c = make(chan uint64)
var m sync.Mutex
for n, i := 0, uint64(0); n < 5; n++ {
go func() {
for {
m.Lock()
i++
v := i
m.Unlock()
if isGold(v) {
c <- v
}
}
}()
}
for n := range c {
fmt.Println("Found gold:", n)
}
}
这个代码定义了一个isGold 的func，可以被65536整除的就是gold，然后定义了一个uint64的channel，接着开了5个go routine,每个go routine都是一个不会停止的协程，加锁i进行自增，并把自增后的i复制给v，如果v可以被65536整除，那么把v放到uint64的channel中，最后打印出channel的值。
如果代码写的不好，并发的时候是很容易出现data race的,运行的时候加上-race这个参数
golang 1.21 运行结果
go run -race demo/concurrency.go
golang version: go1.21.5
Found gold: 65536
Found gold: 131072
Found gold: 196608
Found gold: 262144
Found gold: 327680
Found gold: 393216
Found gold: 458752
Found gold: 524288
Found gold: 589824
Found gold: 655360
^Csignal: interrupt
golang 1.22运行结果
go run -race demo/concurrency.go
golang version: go1.22.1
==================
WARNING: DATA RACE
Write at 0x00c000014148 by goroutine 7:
main.concurrencyDemo.func2()
/home/lfeng/GolandProjects/golang101/demo/concurrency.go:24 +0x65
Previous read at 0x00c000014148 by main goroutine:
main.concurrencyDemo()
/home/lfeng/GolandProjects/golang101/demo/concurrency.go:20 +0x87
main.main()
/home/lfeng/GolandProjects/golang101/demo/concurrency.go:11 +0xc4
Goroutine 7 (running) created at:
main.concurrencyDemo()
/home/lfeng/GolandProjects/golang101/demo/concurrency.go:21 +0x7a
main.main()
/home/lfeng/GolandProjects/golang101/demo/concurrency.go:11 +0xc4
==================
Found gold: 65536
Found gold: 65536
Found gold: 65536
Found gold: 65536
Found gold: 131072
Found gold: 131072
Found gold: 131072
Found gold: 65536
^Csignal: interrupt
可以看到1.21是没有data race的，但是1.22存在了data race
解决方法
把i的定义移到循环外面，修改后的代码
package main
import (
"fmt"
"runtime"
"sync"
)
func main() {
fmt.Println("golang version:", runtime.Version())
concurrencyDemo()
}
func concurrencyDemo() {
isGold := func(num uint64) bool {
return num%65536 == 0
}
var c = make(chan uint64)
var m sync.Mutex
i := uint64(0)
for n := 0; n < 5; n++ {
go func() {
for {
m.Lock()
i++
v := i
m.Unlock()
if isGold(v) {
c <- v
}
}
}()
}
for n := range c {
fmt.Println("Found gold:", n)
}
}
修改后的代码不会存在data race了
再看个例子
package main
import (
"fmt"
"runtime"
"sync"
)
func main() {
fmt.Println("golang version:", runtime.Version())
concurrencyDemo()
}
func concurrencyDemo() {
var wg sync.WaitGroup
wg.Add(5)
for i := 0; i < 5; i++ {
go func() {
defer wg.Done()
fmt.Println(i)
}()
}
}
这个例子其实很简单，使用waitGroup来控制协程的数量和协程的结束。协程中只是打印循环变量i
golang 1.21的运行结果
go run -race demo/concurrency2.go
golang version: go1.21.5
5
5
==================
WARNING: DATA RACE
Read at 0x00c000120028 by goroutine 9:
main.concurrencyDemo.func1()
/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:20 +0xaf
Previous write at 0x00c000120028 by main goroutine:
main.concurrencyDemo()
/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:17 +0xa4
main.main()
/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:11 +0xc4
Goroutine 9 (running) created at:
main.concurrencyDemo()
/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:18 +0x85
main.main()
/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:11 +0xc4
==================
5
5
5
Found 1 data race(s)
exit status 66
golang 1.22运行结果
go run -race demo/concurrency2.go
golang version: go1.22.1
2
1
4
3
0
发现1.21存在data race,但是1.22是不存在data race的
解决方法
go func增加参数传递循环的i
修改后的代码
package main
import (
"fmt"
"runtime"
"sync"
)
func main() {
fmt.Println("golang version:", runtime.Version())
concurrencyDemo()
}
func concurrencyDemo() {
var wg sync.WaitGroup
wg.Add(5)
for i := 0; i < 5; i++ {
go func(i int) {
defer wg.Done()
fmt.Println(i)
}(i)
}
}
修改后的代码也不会存在data race
结论
上面两个例子出现data race的原因都是1.21 loop 变量是初始化一次，但是1.22每次循环都会创建新变量
原文地址：https://blog.csdn.net/wswssx/article/details/137543135
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