SDL Custom Event Registration Limit

Yes, there is a limit, but it's typically very large and unlikely to be a constraint for most applications.

SDL event types are represented by 32-bit unsigned integers (Uint32). SDL reserves a specific range of these IDs for application-defined user events. This range starts at the constant SDL_USEREVENT and ends at SDL_LASTEVENT.

Any event type ID within this range (inclusive) is considered a user event.

The total number of available custom event IDs is determined by the difference between these constants:

Number of IDs = SDL_LASTEVENT - SDL_USEREVENT + 1

Looking at the SDL source code (SDL_events.h), these values are typically defined as:

1#define SDL_USEREVENT   0x8000 /**< User defined event */
2#define SDL_LASTEVENT   0xFFFF /**< This is the last event type */

Using these hexadecimal values (which are 32768 and 65535 in decimal, respectively), the calculation is:

65535 - 32768 + 1 = 32768

So, by default, you can register up to 32,768 distinct custom event types.

This is a substantial number. Most applications will only need a handful, or perhaps a few dozen, custom event types. It's extremely rare for a project to exhaust this limit.

The function SDL_RegisterEvents() requests a number of new, consecutive event type IDs. It returns the ID of the first event type registered, or (Uint32)-1 (which is the maximum value for a Uint32, equivalent to 0xFFFFFFFF) if the request cannot be fulfilled (e.g., if you ask for more IDs than are available).

While unlikely you'll run out of IDs, it's good practice to check the return value, especially if requesting a large number of events at once (which is also uncommon; usually SDL_RegisterEvents(1) is used).

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char* argv[]) {
5  SDL_Init(SDL_INIT_EVENTS);
6
7  Uint32 myEventType = SDL_RegisterEvents(1);
8
9  if (myEventType == (Uint32)-1) {
10    std::cerr << "Error: Could not register"
11                 " custom event type! SDL Error: "
12              << SDL_GetError() << std::endl;
13    SDL_Quit();
14    return 1;
15  }
16
17  std::cout << "Successfully registered event type ID: "
18            << myEventType << std::endl;
19  // myEventType is likely to be 32768 (SDL_USEREVENT)
20  // if this is the first call.
21
22  SDL_Quit();
23  return 0;
24}

In summary, while a theoretical limit exists, it's high enough not to be a practical concern for almost all SDL applications.