Ensuring Thread Safety with Custom Events

How do I ensure thread safety when pushing custom events from multiple threads?

Ensuring thread safety when pushing custom events from multiple threads is crucial to prevent race conditions and undefined behavior. SDL provides thread-safe functions for this purpose, but we need to use them correctly. Here's a guide on how to safely push custom events from multiple threads:

Use SDL_PushEvent()

The SDL_PushEvent() function is thread-safe, meaning it can be safely called from multiple threads without additional synchronization. However, we need to be careful about how we construct and manage the event data.

#include "UserEvents.h"
#include <SDL.h>
#include <thread>
#include <vector>

void ThreadFunction(int threadId) {
  SDL_Event event;
  event.type = UserEvents::CUSTOM_EVENT;
  event.user.code = threadId;
  event.user.data1 = nullptr;
  event.user.data2 = nullptr;

  if (SDL_PushEvent(&event) < 0) {
    SDL_Log("SDL_PushEvent failed: %s\n",
            SDL_GetError());
  }
}

int main() {
  SDL_Init(SDL_INIT_EVERYTHING);

  std::vector<std::thread> threads;
  for (int i = 0; i < 5; ++i) {
    threads.emplace_back(ThreadFunction, i);
  }

  SDL_Event event;
  bool quit{false};

  while (!quit) {
    while (SDL_PollEvent(&event)) {
      if (event.type == SDL_QUIT) {
        quit = true;
      } else if (event.type ==
                 UserEvents::CUSTOM_EVENT) {
        SDL_Log(
            "Received event from thread %d\n",
            event.user.code);
      }
    }
  }

  for (auto& thread : threads) {
    thread.join();
  }

  SDL_Quit();
  return 0;
}

Be Careful with Event Data

While SDL_PushEvent() is thread-safe, we need to be careful about the data we're attaching to the event. If we're using data1 or data2 to point to dynamically allocated memory or shared resources, we need to ensure that:

  1. The memory remains valid until the event is processed.
  2. Access to shared resources is properly synchronized.

Here's an example of how to safely pass data with the event:

#include "UserEvents.h"
#include <SDL.h>
#include <memory>
#include <mutex>
#include <thread>
#include <vector>

struct ThreadData {
  int value;
  std::mutex mutex;
};

void ThreadFunction(int threadId,
                    ThreadData* data) {
  SDL_Event event;
  event.type = UserEvents::CUSTOM_EVENT;
  event.user.code = threadId;
  event.user.data1 = data;

  {
    std::lock_guard<std::mutex> lock(
        data->mutex);
    data->value = threadId * 10;
  }

  if (SDL_PushEvent(&event) < 0) {
    SDL_Log("SDL_PushEvent failed: %s\n",
            SDL_GetError());
  }
}

int main() {
  SDL_Init(SDL_INIT_EVERYTHING);

  auto threadData =
      std::make_unique<ThreadData>();
  std::vector<std::thread> threads;
  for (int i = 0; i < 5; ++i) {
    threads.emplace_back(ThreadFunction, i,
                         threadData.get());
  }

  SDL_Event event;
  bool quit{false};

  while (!quit) {
    while (SDL_PollEvent(&event)) {
      if (event.type == SDL_QUIT) {
        quit = true;
      } else if (event.type ==
                 UserEvents::CUSTOM_EVENT) {
        auto data = static_cast<ThreadData*>(
          event.user.data1);
        int value;
        {
          std::lock_guard<std::mutex> lock(
              data->mutex);
          value = data->value;
        }
        SDL_Log("Received event from thread %d "
                "with value %d\n",
                event.user.code, value);
      }
    }
  }

  for (auto& thread : threads) {
    thread.join();
  }

  SDL_Quit();
  return 0;
}

In this example, we use a mutex to protect access to shared data. The ThreadData struct is allocated in the main thread and remains valid throughout the program's lifetime.

Use SDL_LockMutex() and SDL_UnlockMutex()

For more complex synchronization needs, SDL provides its own mutex functions that are designed to work well with the SDL event system:

#include "UserEvents.h"
#include <SDL.h>
#include <thread>
#include <vector>

SDL_mutex* gMutex = nullptr;
int gSharedValue = 0;

void ThreadFunction(int threadId) {
  SDL_Event event;
  event.type = UserEvents::CUSTOM_EVENT;
  event.user.code = threadId;

  SDL_LockMutex(gMutex);
  gSharedValue += threadId;
  event.user.data1 =
      reinterpret_cast<void*>(gSharedValue);
  SDL_UnlockMutex(gMutex);

  if (SDL_PushEvent(&event) < 0) {
    SDL_Log("SDL_PushEvent failed: %s\n",
            SDL_GetError());
  }
}

int main() {
  SDL_Init(SDL_INIT_EVERYTHING);
  gMutex = SDL_CreateMutex();

  std::vector<std::thread> threads;
  for (int i = 0; i < 5; ++i) {
    threads.emplace_back(ThreadFunction, i);
  }

  SDL_Event event;
  bool quit{false};

  while (!quit) {
    while (SDL_PollEvent(&event)) {
      if (event.type == SDL_QUIT) {
        quit = true;
      } else if (event.type ==
                 UserEvents::CUSTOM_EVENT) {
        SDL_Log("Received event from thread %d "
                "with shared value %d\n",
                event.user.code,
                reinterpret_cast<intptr_t>(
                  event.user.data1));
      }
    }
  }

  for (auto& thread : threads) {
    thread.join();
  }

  SDL_DestroyMutex(gMutex);
  SDL_Quit();
  return 0;
}

By following these practices, you can safely push custom events from multiple threads while avoiding race conditions and ensuring thread safety in your SDL2-based game.

Creating Custom Events

Learn how to create and manage your own game-specific events using SDL's event system.

Questions & Answers

Answers are generated by AI models and may not have been reviewed. Be mindful when running any code on your device.

SDL Custom Event Registration Limit
Is there a limit to how many custom events I can register?
Using Smart Pointers (std::unique_ptr, std::shared_ptr) with SDL Custom Events
Instead of void pointers, can I use std::shared_ptr or std::unique_ptr with data1/data2? How?
Managing Data Lifetime for SDL Custom Events
How do I manage the lifetime of data pointed to by data1/data2 if the event might be processed much later?
SDL Custom Events vs. Building a Separate Event System
Why use SDL_UserEvent instead of just defining my own event system completely separate from SDL's?
Purpose of the code Member in SDL_UserEvent
The code member of SDL_UserEvent wasn't used much. What's its intended purpose?
Comparing SDL Custom Events to Signal/Slot Mechanisms (e.g., Qt)
How does this event system compare to signal/slot mechanisms in frameworks like Qt?
Implementing a Pause/Resume System with Custom Events
How can I use custom events to implement a pause/resume system in my game?
Efficiently Handling Multiple Custom Event Types
What's the best way to handle multiple custom event types without cluttering my main event loop?
Prioritizing Custom Events in SDL
Is there a way to prioritize certain custom events over others in the SDL event queue?
Passing Complex Data in Custom SDL Events
What's the most efficient way to pass complex data structures through custom events?
Or Ask your Own Question
Get an immediate answer to your specific question using our AI assistant