目标C，将Java中的volatile转换为Objective C的最佳方法是什么？

Question

I am trying to convert Java code into Objective C code. And the java code contains variables defined as volatile. I looked online and "volatile" usage in java as follwing

 Essentially, volatile is used to indicate that a variable's value will be modified by different threads.

So, If I was going to set variables as volatile in Objective C because the variables are going to be accessed by different threads then I don't need to set those variables as volatile because I can just set those variables as atomic?

Answer 1

The volatile keyword exists in Objective-C as well. You can use it.

This is because Objective-C is a superset of C.

Declaring the properties as atomic will not correct what volatile was meant to do. volatile effectively tells the compiler not to optimize away checks done on that variable, because it may have changed when the compiler expected it to stay the same.

The simplest example is this. Say we have a global variable declared as:

int packetsReceived = 0;

And it's later used like this:

packetsRecieved = 0;

while (packetsRecieved < 10){
    //Wait for more packets
}

processPackets();

We will never get through that loop, because the compiler will say "Hey, packetsRecieved is never modified in that loop, therefore it will run infinitely." As a result, it will just make it a straight infinite loop so it can avoid having to check every time.

If we instead had declared the variable as:

volatile int packetsRecieved;

We are telling the compiler that this variable may change at any time, even when it looks like it should stay the same. So in our example, the machine code generated by the compiler will still have a check on the condition, and our program will work as expected.

Answer 2

No, atomic and volatile are not the same thing.

atomic (in a property declaration) means that the getter/setter will ensure that a whole value is get/set, regardless of what other threads might be doing simultaneously.

volatile is an indicator to the compiler that a variable can be modified by other means (other threads, memory-mapped IO devices), so it should not optimize out (seemingly) unnecessary loads/stores of that variable. That's similar to what it means in Java, although Java adds some guarantees about memory barriers and ordering of reads and writes that C (and Objective-C) don't provide. In particular, in C, simply declaring a variable volatile is not sufficient to safely use it from multiple threads wikipedia:volatile variable .

Answer 3

Sharing mutable objects between different threads can be a big headache and cause bugs that are difficult to track down. Objective-c has a big preference towards immutable objects. I'd suggest that if possible, you find a way to pass around immutable objects instead if that is possible. Then you don't have to worry about a shared object being written to at all.