65 lines
2.3 KiB
C++
65 lines
2.3 KiB
C++
/*##############################################################################
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## Author: Shaun Reed ##
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## Legal: All Content (c) 2022 Shaun Reed, all rights reserved ##
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## About: An example of a race condition problem and solution ##
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## ##
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## Contact: shaunrd0@gmail.com | URL: www.shaunreed.com | GitHub: shaunrd0 ##
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################################################################################
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*/
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#include <iostream>
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#include <mutex>
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#include <thread>
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#include <vector>
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void problem() {
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std::vector<std::thread> threads;
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const uint8_t thread_count = 5;
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// With no mutex lock, the final value will vary in the range 1000000-5000000
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// + Threads will modify x simultaneously, so some iterations will be lost
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// + x will have same initial value entering this loop on different threads
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uint32_t x = 0;
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for (uint8_t i = 0; i < thread_count; i++) {
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threads.emplace_back([&x](){
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for (uint32_t i = 0; i < 1000000; i++) {
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x = x + 1;
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};
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});
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}
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// Ensure the function doesn't continue until all threads are finished
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// + There's no issue here, the issue is in how `x` is accessed above
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for (auto &thread : threads) thread.join();
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std::cout << x << std::endl;
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}
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// Create mutex lock to prevent threads from modifying same value simultaneously
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static std::mutex mtx;
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void solution() {
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std::vector<std::thread> threads;
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const uint8_t thread_count = 5;
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uint32_t x = 0;
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for (uint8_t i = 0; i < thread_count; i++) {
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threads.emplace_back([&x](){
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// The first thread that arrives here will 'lock' other threads from passing
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// + Once first thread finishes, the next thread will resume
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// + This process repeats until all threads finish
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std::lock_guard<std::mutex> lock(mtx);
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for (uint32_t i = 0; i < 1000000; i++) {
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x = x + 1;
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};
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});
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}
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// Ensure the function doesn't continue until all threads are finished
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for (auto &thread : threads) thread.join();
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std::cout << x << std::endl;
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}
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int main(const int argc, const char * argv[]) {
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// Result will vary from 1000000-5000000
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problem();
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// Result will always be 5000000
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solution();
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return 0;
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}
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