5
type MyMap struct {

     data map[int]int

}
func (m Mymap)foo(){

    //insert  or read from m.data

}
...
   go func f  (m *Mymap){
         for {
            //insert into m.data 
         }
   }()
...
Var m Mymap

m.foo()

When I call m.foo(), as we know , there is a copy of "m",value copy ，which is done by compiler 。 My question is , is there a race in the procedure? It is some kind of reading data from the var "m", I mean , you may need a read lock in case someone is inserting values into m.data when you are copying something from m.data.

If it is thread-safe , is it guarenteed by compiler?

frank.lin
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1 Answers1

2

This is not safe, and there is no implied safe concurrent access in the language. All concurrent data access is unsafe, and needs to be protected with channels or locks.

Because maps internally contain references to the data they contain, even as the outer structure is copied the map still points to the same data. A concurrent map is often a common requirement, and all you need to do is add a mutex to protect the reads and writes. Though a Mutex pointer would work with your value receiver, it's more idiomatic to use a pointer receiver for mutating methods.

type MyMap struct {
    sync.Mutex
    data map[int]int
}

func (m *MyMap) foo() {
    m.Lock()
    defer m.Unlock()
    //insert  or read from m.data
}

The go memory model is very explicit, and races are generally very easy to reason about. When in doubt, always run your program or tests with -race.

JimB
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  • what happen if MyMap has a field foo string or int, or pointer of string or int, does it need lock before assign new value concurrently? I mean you lock it because of field `data` is type map or need to protect all MyMap fields. – Chinglin Wen Feb 18 '21 at 15:36
  • @ChinglinWen: you use the mutex to protect the map, it does not matter what types the map contains. That doesn't mean you may not need additional synchronization elsewhere, but that is outside the scope of the question. – JimB Feb 18 '21 at 15:45