Calling SDL_Init resets Thread Local Storage

[fumehead]

This seems like a totally random behavior. I use thread local storage as a context source that's set at the very beginning of the application, unfortunately calling SDL_Init flushes that entirely, not even calling the destructor that was set.

All SDL threading functionality works as expected without calling SDL_Init first, so I didn't think it was a requirement. Am I doing something wrong?

[fumehead]

OK. I figured it out. It actually takes the TLS ID number 1 without checking whether it belongs to anyone. Just setting my tls id to something else fixes this issue. Is this documented anywhere?

[slouken]

What OS are you using?

[slouken]

The normal use pattern is to set the ID to 0 and it'll be initialized to the next free index when you call SDL_SetTLS()

[fumehead]

My OS is Windows.

The documentation goes
"0 is the invalid ID. An app can create these and then set data for these"

I was expecting there to be a function like "bool CreateTLSID(SDL_TLSID* id)" but since there wasn't one I assumed I need to have my own ID generator and the only constraint there is it being non-zero.

I do see now that there's a comment in SetTLS which I missed
"If the thread local storage ID is not initialized (the value is 0), a new

• ID will be created in a thread-safe way, so all calls using a pointer to
• the same ID will refer to the same local storage."

But it didn't occur to me that a function to store a value for a given key would also be responsible for generating the key in the first place (as long as you give it a pointer to an atomic 0)

[slouken]

Yeah, there isn't a separate function for creating the ID because it prevents race conditions where multiple threads might be setting the data simultaneously.

[fumehead]

I have called SetTLS with my ID of 1. Then SDL_Init went ahead and claimed that, without checking that I've already used it. This is a bug.

[fumehead]

Minimal repro. When I step in with a debugger, my_string is garbage after the delay.

#include "SDL3/SDL_Atomic.h"
#include "SDL3/SDL_Thread.h"
#include "SDL3/SDL_Init.h"
#include "SDL3/SDL_Timer.h"

int main() {
    SDL_TLSID id;
    SDL_SetAtomicInt(&id, 1);
    const char* test = "My string.";
    SDL_SetTLS(&id, (void*)test, nullptr);

    SDL_Init(SDL_INIT_VIDEO);


    SDL_Delay(100);

    const char* my_string = (const char*)SDL_GetTLS(&id);


    return 0;
}

[slouken]

Here's another repro to confirm that the string is valid before init:

#include <stdio.h>
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>

int main(int argc, char *argv[])
{
    SDL_TLSID id;
    SDL_SetAtomicInt(&id, 1);
    const char* test = "My string.";
    SDL_SetTLS(&id, (void*)test, NULL);
    const char* my_stringA = (const char*)SDL_GetTLS(&id);
    printf("String A: %s\n", my_stringA);

    SDL_Init(SDL_INIT_VIDEO);

    SDL_Delay(100);

    const char* my_stringB = (const char*)SDL_GetTLS(&id);
    printf("String B: %s\n", my_stringB);


    return 0;
}

[slouken]

Fixed, thanks!