Golang concurrency: how to append to the same slice from different goroutines

Question

I have concurrent goroutines which want to append a (pointer to a) struct to the same slice. How do you write that in Go to make it concurrency-safe?

This would be my concurrency-unsafe code, using a wait group:

var wg sync.WaitGroup MySlice = make([]*MyStruct) for _, param := range params {     wg.Add(1)     go func(param string) {         defer wg.Done()         OneOfMyStructs := getMyStruct(param)         MySlice = append(MySlice, &OneOfMyStructs)     }(param) } wg.Wait() 

I guess you would need to use go channels for concurrency-safety. Can anyone contribute with an example?

Answer 1

There is nothing wrong with guarding the MySlice = append(MySlice, &OneOfMyStructs) with a sync.Mutex. But of course you can have a result channel with buffer size len(params) all goroutines send their answers and once your work is finished you collect from this result channel.

If your params has a fixed size:

MySlice = make([]*MyStruct, len(params)) for i, param := range params {     wg.Add(1)     go func(i int, param string) {          defer wg.Done()          OneOfMyStructs := getMyStruct(param)          MySlice[i] = &OneOfMyStructs      }(i, param) } 

As all goroutines write to different memory this isn't racy.

Answer 2

The answer posted by @jimt is not quite right, in that it misses the last value sent in the channel and the last defer wg.Done() is never called. The snippet below has the corrections.

https://play.golang.org/p/7N4sxD-Bai

package main  import "fmt" import "sync"  type T int  func main() {     var slice []T     var wg sync.WaitGroup      queue := make(chan T, 1)      // Create our data and send it into the queue.     wg.Add(100)     for i := 0; i < 100; i++ {         go func(i int) {             // defer wg.Done()  <- will result in the last int to be missed in the receiving channel             queue <- T(i)         }(i)     }      go func() {         // defer wg.Done() <- Never gets called since the 100 `Done()` calls are made above, resulting in the `Wait()` to continue on before this is executed         for t := range queue {             slice = append(slice, t)             wg.Done()   // ** move the `Done()` call here         }     }()      wg.Wait()      // now prints off all 100 int values     fmt.Println(slice) }