[cpp] Add example and solution for deadlocks

+ Add example for race condition problem / solution

cpp/CMakeLists.txt

@@ -18,10 +18,12 @@ project(
 )
 
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+add_compile_options("-Wall")
 
 add_subdirectory(algorithms)
 add_subdirectory(cmake-example)
 add_subdirectory(cryptography)
 add_subdirectory(datastructs)
 add_subdirectory(graphics)
+add_subdirectory(multithreading)
 add_subdirectory(patterns)

cpp/algorithms/sorting/bubble/lib-bubble.cpp

@@ -15,9 +15,9 @@
 void BubbleSort(std::vector<int> &array)
 {
   // For each value within the set, starting at 0
-  for (int sortedPivot = 0; sortedPivot < array.size(); sortedPivot++) {
+  for (size_t sortedPivot = 0; sortedPivot < array.size(); sortedPivot++) {
     // Check every other remaining value in the set
-    for (int j = array.size() - 1; j > sortedPivot; j--) {
+    for (size_t j = array.size() - 1; j > sortedPivot; j--) {
       // Swap if the value at j is less than the value before it
       if (array[j] < array[j - 1]) {
         std::swap(array[j], array[j - 1]);

cpp/algorithms/sorting/count/lib-counting.cpp

@@ -33,7 +33,7 @@ void CountingSort(std::vector<int> &array)
 
   // Count the values less than or equal to each element of tempArray
   // + Since each element stores its own count, just add the count at index i-1
-  for (size_t i = 1; i <= maxValue; i++) {
+  for (int32_t i = 1; i <= maxValue; i++) {
     tempArray[i] += tempArray[i - 1];
     // tempArray[i] - 1 now represents the sorted 0-index pos for each value i
   }

cpp/algorithms/sorting/heap/lib-heap.cpp

@@ -17,7 +17,7 @@ size_t Parent(const size_t &index) { return index / 2;}
 size_t Left(const size_t &index) { return 2 * index + 1;}
 size_t Right(const size_t &index) { return (2 * index) + 2;}
 
-void MaxHeapify(std::vector<int> &array, size_t thisIndex, const int &heapSize)
+void MaxHeapify(std::vector<int> &array, size_t thisIndex, const size_t &heapSize)
 {
   // Get an index for the left and right nodes attached to thisIndex
   size_t l = Left(thisIndex);

cpp/algorithms/sorting/heap/lib-heap.hpp

@@ -18,7 +18,7 @@ size_t Parent(const size_t &index);
 size_t Left(const size_t &index);
 size_t Right(const size_t &index);
 
-void MaxHeapify(std::vector<int> &array, size_t thisIndex, const int &heapSize);
+void MaxHeapify(std::vector<int> &array, size_t thisIndex, const size_t &heapSize);
 
 void BuildMaxHeap(std::vector<int> &array);
 

cpp/algorithms/sorting/insertion/lib-insertion.cpp

@@ -15,7 +15,7 @@ void InsertionSort(std::vector<int> &array)
 {
   // For each value, move left until we find sortedPosition for keyValue
   // + Starting with keyValue at array[1], to check sortedPosition at array[0]
-  for (int keyIndex = 1; keyIndex <= array.size(); keyIndex++) {
+  for (size_t keyIndex = 1; keyIndex <= array.size(); keyIndex++) {
     // Save the current key value
     // + We will look for the sorted position of this value
     const int keyValue = array[keyIndex];

cpp/algorithms/sorting/quick/lib-quick.cpp

@@ -50,7 +50,7 @@ size_t Partition(std::vector<int> &array, size_t begin, size_t end)
   // + Return this value when done, so we know where the lhs partition ends
   ssize_t lhsIndex = begin - 1;
   // For each value within this partition, check for values < keyValue
-  for (int j = begin; j <= end - 1; j++) {
+  for (size_t j = begin; j <= end - 1; j++) {
     if (array[j] <= keyValue) {
       // Swap all values < keyValue into the lhs portion of array
       std::swap(array[++lhsIndex], array[j]);

cpp/algorithms/sorting/radix/lib-counting.cpp

@@ -41,7 +41,7 @@ void CountingSort(std::vector<int> &array, int placeValue)
 
   // Count the values less than or equal to each element of tempArray
   // + Since each element stores its own count, just add the count at index i-1
-  for (int i = 1; i < tempArray.size(); i++) {
+  for (size_t i = 1; i < tempArray.size(); i++) {
     tempArray[i] = tempArray[i] + tempArray[i - 1];
   }
 

cpp/algorithms/sorting/selection/lib-select.cpp

@@ -12,10 +12,10 @@
 #include <vector>
 
 void SelectionSort(std::vector<int> &arr) {
-  for (int leftIndex = 0; leftIndex < arr.size(); leftIndex++) {
+  for (size_t leftIndex = 0; leftIndex < arr.size(); leftIndex++) {
     // Get the index for the minimum number in the unsorted set
-    int min = leftIndex;
+    size_t min = leftIndex;
-    for (int i = leftIndex; i < arr.size(); i++) {
+    for (size_t i = leftIndex; i < arr.size(); i++) {
       // Check if value at i is smaller than value at min index
       min = (arr[min] > arr[i]) ? i : min; // Update min value to i if true
     }

cpp/cryptography/columnar-transposition/lib-cipher.cpp

@@ -21,9 +21,9 @@ void Columnar::InitOrder(std::string temp)
   temp.erase(it, temp.end());
 
   // Step through each character in lexicographic order
-  for (int i = 0; i < temp.size(); i++) {
+  for (size_t i = 0; i < temp.size(); i++) {
     // Check each character in the keyWord for the current lexicographic char
-    for (int j = 0; j < keyWord_.size(); j++) {
+    for (size_t j = 0; j < keyWord_.size(); j++) {
       // If they are equal, push the index of the char in keyWord to orderVect
       if (keyWord_[j] == temp[i]) {
         orderVect_.push_back(j);
@@ -109,7 +109,7 @@ std::string Columnar::Decrypt(std::string message)
   rows.resize(orderVect_.size());
   // Track the ending position after each substring is taken
   int lastPos = 0;
-  for (int i = 0; i < orderVect_.size(); i++) {
+  for (size_t i = 0; i < orderVect_.size(); i++) {
     // If we are assigning to any row < fullRows, it should have + 1 character
     if (orderVect_[i] < fullRows) {
       rows[orderVect_[i]] = message.substr(lastPos, rowLength + 1);

cpp/multithreading/CMakeLists.txt

@@ -0,0 +1,19 @@
+################################################################################
+## 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(deadlock)
+add_subdirectory(race-condition)

cpp/multithreading/deadlock/CMakeLists.txt

@@ -0,0 +1,26 @@
+################################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2022 Shaun Reed, all rights reserved                ##
+## About: An example and solution for deadlocks in C++                        ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+
+cmake_minimum_required(VERSION 3.16)
+
+# std::scoped_lock requires C++17
+set(CMAKE_CXX_STANDARD 17)
+add_compile_options("-Wall")
+
+project(
+    #[[NAME]]   Deadlock
+    VERSION     1.0
+    DESCRIPTION "Example and solution for deadlocks in C++"
+    LANGUAGES   CXX
+)
+
+add_executable(
+    multithread-deadlock driver.cpp
+)
+
+target_link_libraries(multithread-deadlock pthread)

cpp/multithreading/deadlock/driver.cpp

@@ -0,0 +1,189 @@
+/*##############################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2022 Shaun Reed, all rights reserved                ##
+## About: An example and solution for deadlocks in C++                        ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+*/
+
+#include <chrono>
+#include <iostream>
+#include <mutex>
+#include <sstream>
+#include <thread>
+
+static std::mutex mtx_A, mtx_B, output;
+
+// Helper function to output thread ID and string associated with mutex name
+// + This must also be thread-safe, since we want threads to produce output
+// + There is no bug or issue here; This is just in support of example output
+void print_safe(const std::string & s) {
+  std::scoped_lock<std::mutex> scopedLock(output);
+  std::cout << s << std::endl;
+}
+
+// Helper function to convert std::thread::id to string
+std::string id_string(const std::thread::id & id) {
+  std::stringstream stream;
+  stream << id;
+  return stream.str();
+}
+
+// In the two threads within this function, we have a problem
+// + The mutex locks are acquired in reverse order, so they collide
+// + This is called a deadlock; The program will *never* finish
+void problem() {
+  std::thread thread_A([]()->void {
+    mtx_A.lock();
+    print_safe(id_string(std::this_thread::get_id()) + " thread_A: Locked A");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+    mtx_B.lock(); // We can't lock B! thread_B is using it
+    // The program will never reach this point in execution; We are in deadlock
+    print_safe(id_string(std::this_thread::get_id())
+               + " thread_A: B has been unlocked, we can proceed!\n  Locked B"
+    );
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id())
+               + " thread_A: Unlocking A, B..."
+    );
+    mtx_A.unlock();
+    mtx_B.unlock();
+  });
+
+  std::thread thread_B([]()->void {
+    mtx_B.lock();
+    print_safe(id_string(std::this_thread::get_id()) + " thread_B: Locked B");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+    mtx_A.lock(); // We can't lock A! thread_A is using it
+    // The program will never reach this point in execution; We are in deadlock
+    print_safe(id_string(std::this_thread::get_id())
+               + " thread_B: A has been unlocked, we can proceed!\n  Locked A"
+    );
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id())
+               + " thread_B: Unlocking B, A..."
+    );
+    mtx_B.unlock();
+    mtx_A.unlock();
+  });
+
+  // This offers a way out of the deadlock, so we can proceed to the solution
+  std::this_thread::sleep_for(std::chrono::seconds(2));
+  char input;
+  print_safe("\n"
+             + id_string(std::this_thread::get_id())
+             + " problem(): We are in a deadlock. \n"
+             + "    Enter y/Y to continue to the solution...\n"
+  );
+  while (std::cin >> input) {
+    if (input != 'Y' && input != 'y') continue;
+    else break;
+  }
+  print_safe(id_string(std::this_thread::get_id())
+             + " problem(): Unlocking A, B..."
+  );
+  mtx_A.unlock();
+  mtx_B.unlock();
+
+  thread_A.join();
+  thread_B.join();
+}
+
+// std::lock will lock N mutex locks
+// + If either is in use, execution will block until both are available to lock
+void solution_A() {
+  std::thread thread_A([]()->void {
+    std::lock(mtx_A, mtx_B);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked A, B");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking A, B...");
+    mtx_A.unlock();
+    mtx_B.unlock();
+  });
+
+  std::thread thread_B([]()->void {
+    std::lock(mtx_B, mtx_A);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked B, A");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking B, A...");
+    mtx_B.unlock();
+    mtx_A.unlock();
+  });
+
+  thread_A.join();
+  thread_B.join();
+}
+
+// std::lock_guard is a C++11 object which can be constructed with 1 mutex
+// + When the program leaves the scope of the guard, the mutex is unlocked
+void solution_B() {
+  std::thread thread_A([]()->void {
+    // lock_guard will handle unlocking when program leaves this scope
+    std::lock_guard<std::mutex> guard_A(mtx_A), guard_B(mtx_B);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked A, B");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking A, B...");
+    // We don't need to explicitly unlock either mutex
+  });
+
+  std::thread thread_B([]()->void {
+    std::lock_guard<std::mutex> guard_B(mtx_B), guard_A(mtx_A);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked B, A");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking B, A...");
+    // We don't need to explicitly unlock either mutex
+  });
+
+  thread_A.join();
+  thread_B.join();
+}
+
+// std::scoped_lock is a C++17 object that can be constructed with N mutex
+// + When the program leaves this scope, all N mutex will be unlocked
+void solution_C() {
+  std::thread thread_A([]()->void {
+    // scoped_lock will handle unlocking when program leaves this scope
+    std::scoped_lock scopedLock(mtx_A, mtx_B);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked A, B");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking A, B...");
+    // We don't need to explicitly unlock either mutex
+  });
+
+  std::thread thread_B([]()->void {
+    std::scoped_lock scopedLock(mtx_A, mtx_B);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked B, A");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking B, A...");
+    // We don't need to explicitly unlock either mutex
+  });
+
+  thread_A.join();
+  thread_B.join();
+}
+
+int main(const int argc, const char * argv[]) {
+  std::cout << "main() thread id: " << std::this_thread::get_id() << std::endl;
+
+  problem();
+
+  print_safe("\nsolution_A, using std::lock\n");
+  solution_A();
+
+  print_safe("\nsolution_B, using std::lock_guard\n");
+  solution_B();
+
+  print_safe("\nsolution_C, using std::scoped_lock\n");
+  solution_C();
+
+  return 0;
+}

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;
+}