Is a struct actually copied between goroutines if sent over a Golang channel?

Issue

If a large struct is sent over a channel in Go, is it actually copied between goroutines?

For example, in the code below, will Go actually copy all largeStruct data between goroutines producer and consumer?

package main

import (
    "fmt"
    "sync"
)

type largeStruct struct {
    buf [10000]int
}

func main() {
    ch := make(chan largeStruct)
    wg := &sync.WaitGroup{}
    wg.Add(2)
    go consumer(wg, ch)
    go producer(wg, ch)
    wg.Wait()
}

func producer(wg *sync.WaitGroup, output chan<- largeStruct) {
    defer wg.Done()
    for i := 0; i < 5; i++ {
        fmt.Printf("producer: %d\n", i)
        output <- largeStruct{}
    }
    close(output)
}

func consumer(wg *sync.WaitGroup, input <-chan largeStruct) {
    defer wg.Done()
    i := 0
LOOP:
    for {
        select {
        case _, ok := <-input:
            if !ok {
                break LOOP
            }
            fmt.Printf("consumer: %d\n", i)
            i++
        }
    }
}

Playground: http://play.golang.org/p/fawEQnSDwB

Solution

Yes, everything is a copy in Go, you can easily work around that by changing the channel to use a pointer (aka chan *largeStruct).

// demo: http://play.golang.org/p/CANxwt8s2B

As you can see, the pointer to v.buf is different in each case, however if you change it to chan *largeStruct, the pointers will be the same.

@LucasJones provided a little easier to follow example: https://play.golang.org/p/-VFWCgOnh0

As @nos pointed out, there’s a potential race if you modify the value in both goroutines after sending it.

Answered By – OneOfOne

Answer Checked By – Mary Flores (GoLangFix Volunteer)

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