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runtime: value copy issue #29626

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sunvim opened this issue Jan 9, 2019 · 3 comments

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@sunvim
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commented Jan 9, 2019

What version of Go are you using (go version)?

go 1.10.3 , go 1.10.4 , go 1.11

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

windows 10 x86_64

What did you do?

package main

import "fmt"

type Person struct {
Name string
Age int
}

func main() {
m := make(map[string]Person, 2)
m["1"] = Person{"n1", 11}
m["2"] = Person{"n2", 12}
for _, v := range m {
go output(&v)
}
select {}
}

func output(v *Person) {
fmt.Println(v)
}

What did you expect to see?

&{n1,11}
&{n2,12}

What did you see instead?

&{n2 12}
&{n2 12}

@agnivade

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commented Jan 9, 2019

Please see - https://github.com/golang/go/wiki/CommonMistakes#using-goroutines-on-loop-iterator-variables

To fix it, use -

go func(v Person){output(&v)}(v)

@agnivade agnivade closed this Jan 9, 2019

@sunvim

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commented Jan 10, 2019

Please see - https://github.com/golang/go/wiki/CommonMistakes#using-goroutines-on-loop-iterator-variables

To fix it, use -

go func(v Person){output(&v)}(v)

yes, I know how to fix it.

but pass value by this way, the perfermance maybe a bit effect.

@mattn

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commented Jan 10, 2019

Then, please try this.

func main() {
	m := make(map[string]Person, 2)
	m["1"] = Person{"n1", 11}
	m["2"] = Person{"n2", 12}
	for _, v := range m {
		v := v // ADD THIS
		go output(&v)
	}
	select {}
}
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