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