[cpp] Add multithreaded project

+ Add example for race condition problem / solution
master
Shaun Reed 2 years ago
parent fc1f247987
commit d81c65b1d2

@ -25,4 +25,5 @@ add_subdirectory(cmake-example)
add_subdirectory(cryptography)
add_subdirectory(datastructs)
add_subdirectory(graphics)
add_subdirectory(patterns)
add_subdirectory(multithreading)
add_subdirectory(patterns)

@ -0,0 +1,18 @@
################################################################################
## Author: Shaun Reed ##
## Legal: All Content (c) 2022 Shaun Reed, all rights reserved ##
## About: A root project for practicing C++ multithreading ##
## ##
## Contact: shaunrd0@gmail.com | URL: www.shaunreed.com | GitHub: shaunrd0 ##
################################################################################
cmake_minimum_required(VERSION 3.16)
project(
#[[NAME]] Multithreading
VERSION 1.0
DESCRIPTION "Practice with multithreaded programming in C++"
LANGUAGES CXX
)
add_subdirectory(race-condition)

@ -0,0 +1,22 @@
################################################################################
## Author: Shaun Reed ##
## Legal: All Content (c) 2022 Shaun Reed, all rights reserved ##
## About: An example and solution for race conditions in C++ ##
## ##
## Contact: shaunrd0@gmail.com | URL: www.shaunreed.com | GitHub: shaunrd0 ##
################################################################################
cmake_minimum_required(VERSION 3.16)
project(
#[[NAME]] RaceCondition
VERSION 1.0
DESCRIPTION "Example and solution for race conditions"
LANGUAGES CXX
)
add_executable(
multithread-race-condition driver.cpp
)
target_link_libraries(multithread-race-condition pthread)

@ -0,0 +1,64 @@
/*##############################################################################
## 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;
}
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