多线程中的变量访问，不同步

Question

I have following code

int i; //gobal var

Thread1:
{
...
  i=some value;
}

Thread2:
{
  if (i==2) dosomething();
  else dosomethingelse();
  i = 4;
}

I want to write it to be thread safe without using synchronization objects and in C++ standard way.

my questions is how to have a variable to read/write access by different threads with out using synchronization? my requirement is have a bool variable which can have true or false.

Is volatile variable is atomic.

Please note that i am not supposed to use any libraires like TBB which has atomic variable.

Reason for asking this question we don't want to take and release semphore every time we access the variable in thread, as this variable changed not very often.

Answer 1

Well your code does not have any meaning right now. Becaues you are basically saying`

Thread2:
{
  if (i==2) dosomething();
  //let's do something if i is 2 at the moment I am reading it.
  else dosomethingelse();
  i = 4;
}`

It is ok. But if you are doing something with i then you are in trouble. If not, and if you are asking whether accessing i is atomic or not, it depends on cpu architecture. For example for x86 and x64 it is atomic. Read this .