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What is a race condition?

When writing multithreaded applications, one of the most common problems experienced is race conditions.

My questions to the community are:

  • What is the race condition?
  • How do you detect them?
  • How do you handle them?
  • Finally, how do you prevent them from occurring?
like image 582
bmurphy1976 Avatar asked Aug 29 '08 15:08

bmurphy1976


People also ask

What is race condition?

A race condition is an undesirable situation that occurs when a device or system attempts to perform two or more operations at the same time, but because of the nature of the device or system, the operations must be done in the proper sequence to be done correctly.

What is race condition with example?

A race condition is a condition when there are many processes and every process shares the data with each other and accessing the data concurrently, and the output of execution depends on a particular sequence in which they share the data and access.

What is a race condition in Java?

Race condition in Java is a type of concurrency bug or issue that is introduced in your program because of parallel execution of your program by multiple threads at the same time, Since Java is a multi-threaded programming language hence the risk of Race condition is higher in Java which demands a clear understanding ...

How do you fix race conditions?

Race conditions can be avoided by proper thread synchronization in critical sections. Thread synchronization can be achieved using a synchronized block of Java code. Thread synchronization can also be achieved using other synchronization constructs like locks or atomic variables like java.


1 Answers

A race condition occurs when two or more threads can access shared data and they try to change it at the same time. Because the thread scheduling algorithm can swap between threads at any time, you don't know the order in which the threads will attempt to access the shared data. Therefore, the result of the change in data is dependent on the thread scheduling algorithm, i.e. both threads are "racing" to access/change the data.

Problems often occur when one thread does a "check-then-act" (e.g. "check" if the value is X, then "act" to do something that depends on the value being X) and another thread does something to the value in between the "check" and the "act". E.g:

if (x == 5) // The "Check" {    y = x * 2; // The "Act"     // If another thread changed x in between "if (x == 5)" and "y = x * 2" above,    // y will not be equal to 10. } 

The point being, y could be 10, or it could be anything, depending on whether another thread changed x in between the check and act. You have no real way of knowing.

In order to prevent race conditions from occurring, you would typically put a lock around the shared data to ensure only one thread can access the data at a time. This would mean something like this:

// Obtain lock for x if (x == 5) {    y = x * 2; // Now, nothing can change x until the lock is released.                // Therefore y = 10 } // release lock for x 
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Lehane Avatar answered Oct 12 '22 21:10

Lehane