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

Question

Ryan McCombe · Accepted Answer

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.

1#include "UserEvents.h"
2#include <SDL.h>
3#include <thread>
4#include <vector>
5
6void ThreadFunction(int threadId) {
7  SDL_Event event;
8  event.type = UserEvents::CUSTOM_EVENT;
9  event.user.code = threadId;
10  event.user.data1 = nullptr;
11  event.user.data2 = nullptr;
12
13  if (SDL_PushEvent(&event) < 0) {
14    SDL_Log("SDL_PushEvent failed: %s\n",
15            SDL_GetError());
16  }
17}
18
19int main() {
20  SDL_Init(SDL_INIT_EVERYTHING);
21
22  std::vector<std::thread> threads;
23  for (int i = 0; i < 5; ++i) {
24    threads.emplace_back(ThreadFunction, i);
25  }
26
27  SDL_Event event;
28  bool quit{false};
29
30  while (!quit) {
31    while (SDL_PollEvent(&event)) {
32      if (event.type == SDL_QUIT) {
33        quit = true;
34      } else if (event.type ==
35                 UserEvents::CUSTOM_EVENT) {
36        SDL_Log(
37            "Received event from thread %d\n",
38            event.user.code);
39      }
40    }
41  }
42
43  for (auto& thread : threads) {
44    thread.join();
45  }
46
47  SDL_Quit();
48  return 0;
49}

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:

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

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

1#include "UserEvents.h"
2#include <SDL.h>
3#include <memory>
4#include <mutex>
5#include <thread>
6#include <vector>
7
8struct ThreadData {
9  int value;
10  std::mutex mutex;
11};
12
13void ThreadFunction(int threadId,
14                    ThreadData* data) {
15  SDL_Event event;
16  event.type = UserEvents::CUSTOM_EVENT;
17  event.user.code = threadId;
18  event.user.data1 = data;
19
20  {
21    std::lock_guard<std::mutex> lock(
22        data->mutex);
23    data->value = threadId * 10;
24  }
25
26  if (SDL_PushEvent(&event) < 0) {
27    SDL_Log("SDL_PushEvent failed: %s\n",
28            SDL_GetError());
29  }
30}
31
32int main() {
33  SDL_Init(SDL_INIT_EVERYTHING);
34
35  auto threadData =
36      std::make_unique<ThreadData>();
37  std::vector<std::thread> threads;
38  for (int i = 0; i < 5; ++i) {
39    threads.emplace_back(ThreadFunction, i,
40                         threadData.get());
41  }
42
43  SDL_Event event;
44  bool quit{false};
45
46  while (!quit) {
47    while (SDL_PollEvent(&event)) {
48      if (event.type == SDL_QUIT) {
49        quit = true;
50      } else if (event.type ==
51                 UserEvents::CUSTOM_EVENT) {
52        auto data = static_cast<ThreadData*>(
53          event.user.data1);
54        int value;
55        {
56          std::lock_guard<std::mutex> lock(
57              data->mutex);
58          value = data->value;
59        }
60        SDL_Log("Received event from thread %d "
61                "with value %d\n",
62                event.user.code, value);
63      }
64    }
65  }
66
67  for (auto& thread : threads) {
68    thread.join();
69  }
70
71  SDL_Quit();
72  return 0;
73}

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:

1#include "UserEvents.h"
2#include <SDL.h>
3#include <thread>
4#include <vector>
5
6SDL_mutex* gMutex = nullptr;
7int gSharedValue = 0;
8
9void ThreadFunction(int threadId) {
10  SDL_Event event;
11  event.type = UserEvents::CUSTOM_EVENT;
12  event.user.code = threadId;
13
14  SDL_LockMutex(gMutex);
15  gSharedValue += threadId;
16  event.user.data1 =
17      reinterpret_cast<void*>(gSharedValue);
18  SDL_UnlockMutex(gMutex);
19
20  if (SDL_PushEvent(&event) < 0) {
21    SDL_Log("SDL_PushEvent failed: %s\n",
22            SDL_GetError());
23  }
24}
25
26int main() {
27  SDL_Init(SDL_INIT_EVERYTHING);
28  gMutex = SDL_CreateMutex();
29
30  std::vector<std::thread> threads;
31  for (int i = 0; i < 5; ++i) {
32    threads.emplace_back(ThreadFunction, i);
33  }
34
35  SDL_Event event;
36  bool quit{false};
37
38  while (!quit) {
39    while (SDL_PollEvent(&event)) {
40      if (event.type == SDL_QUIT) {
41        quit = true;
42      } else if (event.type ==
43                 UserEvents::CUSTOM_EVENT) {
44        SDL_Log("Received event from thread %d "
45                "with shared value %d\n",
46                event.user.code,
47                reinterpret_cast<intptr_t>(
48                  event.user.data1));
49      }
50    }
51  }
52
53  for (auto& thread : threads) {
54    thread.join();
55  }
56
57  SDL_DestroyMutex(gMutex);
58  SDL_Quit();
59  return 0;
60}

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

Ensuring Thread Safety with Custom Events

Use SDL_PushEvent()

Be Careful with Event Data

Use SDL_LockMutex() and SDL_UnlockMutex()

Creating Custom Events

Game Development with SDL2

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