Is it thread safe to access different members of struct in go?

Question

Is it safe to access different struct members from different goroutines?

I understand that writing to the same variable without sync is dangareous:

package main  type Apple struct {     color string     size  uint }  func main() {     apple := &Apple{}     go func() {         apple.color = "red"     }()     go func() {         apple.color = "green"     }() } 

But can you write to different struct members without any kind of synchronization?

package main  type Apple struct {     color string     size  uint }  func main() {     apple := &Apple{}     go func() {         apple.color = "red"     }()     go func() {         apple.size = 42     }() } 

Or should I use chan or sync.Mutex for that?

Answer 1

It should be safe to access different variables from different threads, and struct members are different variables. So yes it should be safe.

However, it may not be fast. Variables that are close together in memory like struct members are will share a CPU cache line. A cache line is the smallest piece of memory that a CPU (well, most current models) can lock. That means that CPU-2 has to wait to write until CPU-1 has finished with that cache line even if they are writing to different variables.

It is NOT safe to change the pointer to the struct while writing to the struct from different threads. In your example if you had a third goroutine that did apple = &Apple{} some of the other goroutines in other threads might write to the old Apple or the new Apple and you wouldn't know.