Advanced Programming Techniques in D/

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A Race Condition Example

A Race Condition Example

Let’s look at a race condition example.

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The correctness of the program requires extra attention when mutable data is shared between threads.

Example

To see an example of a race condition, let’s consider multiple threads sharing the same mutable variable. The threads in the following program receive the addresses as two variables and swap their values a large number of times:

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import std.stdio;

import std.concurrency;

import core.thread;



void swapper(shared(int) * first, shared(int) * second) {

    foreach (i; 0 .. 10_000) {

        int temp = *second;

        *second = *first;

        *first = temp;

    }

}



void main() {

    shared(int) i = 1;

    shared(int) j = 2;



    writefln("before: %s and %s", i, j);



    foreach (id; 0 .. 10) {

        spawn(&swapper, &i, &j);

    }



    // Wait for all threads to finish their tasks

    thread_joinAll();



    writefln("after : %s and %s", i, j);

}

Although the program above gets compiled successfully, in most cases it would work incorrectly. Observe that it starts ten threads that all access the same two variables i and j. As a result ...