Always call Add() before spawning the corresponding goroutine(s). And
always call Add() in the same goroutine that also calls Wait(). This way
you have no race condition between Add(), Done(), and Wait().
On Saturday, November 4, 2017 at 10:46:21 AM UTC+1, Henrik Johansson wrote:
>
> I find continuously adding and calling done on a waitgroup a bit odd. The
> waiting goroutine can continue as soon as the count is zero so there is a
> race between adds and dones.
>
> On Sat, 4 Nov 2017, 10:01 , <28911...@gmail.com <javascript:>> wrote:
>
>> Thank you very much.Sorry I still have some question:what's the function
>> of wg? what does the sentense "wg.Add(2) " mean?? I found that if I change
>> 2 into 1,the result is differnet.
>>
>> 在 2017年11月3日星期五 UTC+8上午4:45:47，snmed写道：
>>
>>> Hi
>>>
>>> Here is the code:
>>>
>>> Version 1:
>>> package main
>>>
>>> import (
>>> "fmt"
>>> "sync"
>>> )
>>>
>>> var a string
>>> var once sync.Once
>>>
>>> func setup() {
>>> a = "hello,world\n"
>>> }
>>> func doprint() {
>>> once.Do(setup)
>>> fmt.Print(a)
>>> }
>>> func twoprint() <-chan struct{} {
>>> var wg sync.WaitGroup
>>> wg.Add(2)
>>> ch := make(chan struct{})
>>>
>>> go func() {
>>> doprint()
>>> wg.Done()
>>> }()
>>> go func() {
>>> doprint()
>>> wg.Done()
>>> }()
>>>
>>> go func() {
>>> wg.Wait()
>>> close(ch)
>>> }()
>>>
>>> return ch
>>> }
>>>
>>> func main() {
>>> ch := twoprint()
>>> <-ch
>>> }
>>>
>>>
>>> Version 2:
>>> package main
>>>
>>> import (
>>> "fmt"
>>> "sync"
>>> )
>>>
>>> var a string
>>> var once sync.Once
>>>
>>> func setup() {
>>> a = "hello,world\n"
>>> }
>>> func doprint() {
>>> once.Do(setup)
>>> fmt.Print(a)
>>> }
>>> func twoprint() {
>>> var wg sync.WaitGroup
>>> wg.Add(2)
>>>
>>> go func() {
>>> doprint()
>>> wg.Done()
>>> }()
>>> go func() {
>>> doprint()
>>> wg.Done()
>>> }()
>>>
>>> wg.Wait()
>>> }
>>>
>>> func main() {
>>> twoprint()
>>> }
>>>
>>>
>>> Cheers snmed
>>>
>>>
>>> Am Donnerstag, 2. November 2017 10:37:15 UTC+1 schrieb
>>> 28911...@gmail.com:
>>>>
>>>> Sorry,I try my best to open the website but it can't work.Can you write
>>>> it ??Thank you so much.
>>>>
>>>> 在 2017年10月30日星期一 UTC+8下午4:29:44，snmed写道：
>>>>>
>>>>> Hi
>>>>>
>>>>> There are several ways to solve it, here are two of them:
>>>>>
>>>>> https://play.golang.org/p/wJwkI7HQwv
>>>>> https://play.golang.org/p/nasUcgBeG4
>>>>>
>>>>> I prefer the first one, because so I can decide if i want to wait for
>>>>> the end of twoprint or not.
>>>>>
>>>>> Cheers
>>>>>
>>>>> Am Montag, 30. Oktober 2017 06:43:45 UTC+1 schrieb 28911...@gmail.com:
>>>>>>
>>>>>> Yes, you're right.So how to solve it??
>>>>>>
>>>>>> 在 2017年10月30日星期一 UTC+8下午12:37:49，Dave Cheney写道：
>>>>>>>
>>>>>>> Hello. I’m guessing that your tried calling twoprint(), but you’ve
>>>>>>> probably found that nothing is printed to the screen before your
>>>>>>> program
>>>>>>> exits.
>>>>>>>
>>>>>>> Is that correct?
>>>>>>
>>>>>> --
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>>
>
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