[cpp] Add multithreaded project

+ Add example for race condition problem / solution

cpp/CMakeLists.txt

@@ -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)

cpp/multithreading/CMakeLists.txt

@@ -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)

cpp/multithreading/race-condition/CMakeLists.txt

@@ -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)

cpp/multithreading/race-condition/driver.cpp

@@ -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;
+}