On Thu, Nov 3, 2016 at 10:28 PM, 刘桂祥 <liuguixiang...@gmail.com> wrote: > > In my write goroutine I don't modify the map key (data structure) but set > the variable point to another map address ,this is a pointer assignment and > is atomic
You're right, my previous reply was incorrect. My apologies. The real reason your code is incorrect is more subtle. A map is a pointer to a complex data structure. Setting values in a map is a series of memory writes. From the point of view of the processor doing the writes, they all appear in order. From the pointer of view of a different processor, they need not. It is possible the the other processor to see the final write, to gMap, before it sees the other writes, setting the values in the map. The read from gMap will then be trying to access an incomplete data structure. In order to force all the writes to be visible to the reading processor, you need to use a sync.Mutex, a channel operation, or a sync/atomic.Store function. There is a simple rule to follow when it comes to communicating between processes: always use channels or locks. Don't try to be clever. Please read https://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong. Ian > 在 2016年11月4日星期五 UTC+8下午1:22:20，Ian Lance Taylor写道： >> >> On Thu, Nov 3, 2016 at 10:19 PM, 刘桂祥 <liuguix...@gmail.com> wrote: >> > can you explain why whis ? >> >> A map is basically a pointer to a complex data structure. Setting a >> value in a map changes that data structure. If one goroutine is >> reading from the data structure while a different goroutine is writing >> to the data structure, the results are completely unpredictable. In >> the worst case they could even cause the program to crash. >> >> Ian >> >> >> > 在 2016年11月4日星期五 UTC+8下午1:16:39，Ian Lance Taylor写道： >> >> >> >> On Thu, Nov 3, 2016 at 8:37 PM, 刘桂祥 <liuguix...@gmail.com> wrote: >> >> > // example.go >> >> > >> >> > package main >> >> > >> >> > var gMap = make(map[int]int) >> >> > >> >> > func w() { >> >> > temp := make(map[int]int) >> >> > temp[1] = 100 >> >> > temp[2] = 200 >> >> > gMap = temp // Does the compiler or cpu will reorder >> >> > temp[1]=100, >> >> > temp[2]=200, gMap=temp ?? >> >> > } >> >> > >> >> > func r() { >> >> > local := gMap >> >> > println(local[1], local[2]) >> >> > } >> >> > >> >> > func main() { >> >> > >> >> > go w() >> >> > go r() >> >> > >> >> > // ... >> >> > } >> >> > >> >> > I have one goroutine to read the map and one goroutine to rewrite the >> >> > global >> >> > map variable does this safe ?? >> >> >> >> No. Use a lock. >> >> >> >> Ian >> > >> > -- >> > You received this message because you are subscribed to the Google >> > Groups >> > "golang-nuts" group. >> > To unsubscribe from this group and stop receiving emails from it, send >> > an >> > email to golang-nuts...@googlegroups.com. >> > For more options, visit https://groups.google.com/d/optout. > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
