Output Streams in Multithreading

Using C++ output streams in multithreaded applications can be challenging due to potential issues with thread safety and output interleaving. Here are some common pitfalls and how to avoid them.

Output Interleaving

When multiple threads write to the same output stream simultaneously, their outputs can become interleaved, leading to jumbled and unreadable text. For example:

1#include <iostream>
2#include <thread>
3
4void printMessage(const std::string& message) {
5  std::cout << message << '\n';  
6}
7
8int main() {
9  std::thread t1(
10    printMessage, "Hello from thread 1");
11  std::thread t2(
12    printMessage, "Hello from thread 2");
13  t1.join();
14  t2.join();
15}

1Hello from thread 1
2Hello from thread 2

Outputs might not appear in order, or lines might get mixed up:

1Hello from thread 2
2Hello from thread 1

Using Mutex for Synchronization

To prevent interleaving, use a mutex to synchronize access to the output stream:

1#include <iostream>
2#include <thread>
3#include <mutex>
4
5std::mutex coutMutex;
6
7void printMessage(const std::string& message) {
8  std::lock_guard<std::mutex> guard(coutMutex);
9  std::cout << message << '\n';
10}
11
12int main() {
13  std::thread t1(
14    printMessage, "Hello from thread 1");
15
16  // Ensure t1 completes before starting t2
17  t1.join();
18
19  std::thread t2(
20    printMessage, "Hello from thread 2");
21
22  // Ensure t2 completes before exiting main
23  t2.join();
24}

1Hello from thread 1
2Hello from thread 2

Here, std::lock_guard ensures that only one thread accesses std::cout at a time, preventing interleaving.

Performance Considerations

Locking a mutex for every output operation can impact performance. To mitigate this, consider minimizing the number of lock operations:

1#include <iostream>
2#include <thread>
3#include <mutex>
4#include <string>
5
6std::mutex coutMutex;
7
8void printNumbers(int start) {
9  std::string output;
10  for (int i = start; i < start + 10; ++i) {
11    output += std::to_string(i) + ' ';
12  }
13
14  std::lock_guard<std::mutex> guard(coutMutex);  
15  std::cout << output << '\n';
16}
17
18int main() {
19  std::thread t1(printNumbers, 0);
20  std::thread t2(printNumbers, 10);
21  t1.join();
22  t2.join();
23}

10 1 2 3 4 5 6 7 8 9
210 11 12 13 14 15 16 17 18 19

In this example, the mutex is locked only once per thread, reducing the performance impact.

Avoiding Deadlocks

Deadlocks can occur if threads try to acquire locks in different orders. To avoid deadlocks, always acquire locks in the same order across all threads.

Using std::cerr for Error Messages

std::cerr is unbuffered and can be used for error messages in multithreaded applications. However, you should still use a mutex to prevent interleaving.

Summary

When using C++ output streams in multithreaded applications, avoid common pitfalls by:

• Using a mutex to prevent output interleaving
• Minimizing the number of lock operations for performance
• Consistently ordering lock acquisition to avoid deadlocks

By following these practices, you can ensure clear and thread-safe output in your multithreaded C++ applications.