Compare Exchange Operation

The compare-exchange operation is a fundamental concept in parallel computing and multi-threading. It atomically compares the value of a variable with a given value, and if they match, it updates the variable with a new value. This operation is used for tasks such as synchronization, implementing locks, and building certain data structures like linked lists or queues in a thread-safe manner. The key takeaway is that compare-exchange ensures atomicity and is crucial for writing concurrent programs.

Java Code

Java provides built-in support for atomic operations through classes like AtomicInteger. Here’s how you can implement a compare-exchange:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

import java.util.concurrent.atomic.AtomicInteger;

public class CompareExchangeDemo {
    public static void main(String[] args) {
        AtomicInteger value = new AtomicInteger(0);

        int expected = 0;
        int update = 1;
        value.compareAndSet(expected, update);
    }
}
  1. We create an AtomicInteger called value and initialize it to 0.
  2. compareAndSet(expected, update) checks if value is equal to expected; if so, it sets value to update.

C++ Code

In C++, std::atomic can be used for compare-exchange:

1
2
3
4
5
6
7
8
9

#include <atomic>

int main() {
    std::atomic<int> value(0);
    int expected = 0;
    int update = 1;

    value.compare_exchange_strong(expected, update);
}
  1. We declare an std::atomic<int> called value.
  2. compare_exchange_strong(expected, update) compares value to expected and updates it if they match.

Python Code

Python’s threading library doesn’t natively support atomic compare-and-exchange, but you can use locks to simulate this behavior:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10

import threading

value = 0
lock = threading.Lock()
expected = 0
update = 1

with lock:
    if value == expected:
        value = update
  1. A variable value is declared and initialized to 0.
  2. We use threading.Lock() to ensure that the comparison and update are atomic.

In all three implementations, the basic idea remains the same: compare a value atomically and update it if the condition is met. This serves as a foundation for more complex operations in multi-threaded programming.