I am trying to store a map as unsafe.Pointer and trying to retrieve it later. Below is the code snippet:
package main
import (
"fmt"
"sync/atomic"
"unsafe"
)
type MapHolder struct {
ptr unsafe.Pointer
}
func (m MapHolder) Set(myMap map[string]int64) {
atomic.StorePointer(&m.ptr, unsafe.Pointer(&myMap))
fmt.Printf("The pointer value is: %v
", atomic.LoadPointer(&m.ptr))
}
func (m MapHolder) Get() map[string]int64 {
ptr := atomic.LoadPointer(&m.ptr)
if ptr == nil {
fmt.Printf("Why is this pointer value nil?")
return nil
} else {
return *(*map[string]int64)(ptr)
}
}
func main() {
var m MapHolder
test := make(map[string]int64)
test["hello"] = 1
m.Set(test)
m.Get()
}
When Set(), executes, it prints the contents of m.ptr correctly. However, when we do a Get(), it returns nil. This looks totally un-expected to me. I am using go 1.11.6
The output of the above program is:
The pointer loaded is: 0x40c130
Why is this pointer value nil?
Because the Set method has a value receiver, the receiver value and any changes to the value are discarded when the method returns. Fix the problem by using pointer receivers.
func (m *MapHolder) Set(myMap map[string]int64) { ... }
func (m *MapHolder) Get() map[string]int64 { ... }
The Get method must also use a pointer receiver to prevent a data race on the copy of the receiver argument.
The Set method stores the address of argument myMap, not the address of m in main. This may or may not be a problem depending on how the application uses MapHolder.