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Add an invalid #define for SDL_ThreadID() to SDL_oldnames.h (#15801)

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in SDL2 SDL_ThreadID() was a function that's now
SDL_GetCurrentThreadID(), but in SDL3 SDL_ThreadID is a type, so
in C++ `x = SDL_ThreadID()` is valid code (default constructor which
in case of integers means 0), so that's a massive footgun.

See the big comment in SDL_oldnames.h for more details.

Added `#undef SDL_ThreadID` in SDL_dynapi.c because it has one of the
(quite rare) cases  where "SDL_ThreadID" followed by a "(" is actually
correct and necessary (function pointer returning SDL_ThreadID).
This commit is contained in:
Daniel Gibson
2026-06-12 19:24:56 +02:00
committed by GitHub
parent 4f031ea5da
commit b04d026458
3 changed files with 71 additions and 0 deletions
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54
include/SDL3/SDL_oldnames.h
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@@ -1336,4 +1336,58 @@
#endif /* SDL_ENABLE_OLD_NAMES */
/* In SDL2, `SDL_ThreadID()` was a function that is now called `SDL_GetCurrentThreadID()`.
* In SDL3, the thread ID *type* is called `SDL_ThreadID` (in SDL2 it was
* `SDL_threadID` with lower 't').
*
* Unfortunately, at least in C++ writing SDL_ThreadID() compiles fine even for
* the type, it's the default constructor of the type, so e.g.
* `myID = SDL_ThreadID();` is equivalent to `myID = 0;`
*
* Of course if you've been porting SDL2 code and have missed renaming this case
* of `SDL_ThreadID()` to `SDL_GetCurrentThreadID()` this is quite a pitfall:
* The code compiles fine, but behaves wrong: it will set `myID` to `0` instead
* of the current thread's ID.
*
* This makes it impossible to provide a "proper" `SDL_ENABLE_OLD_NAMES` treatment
* of SDL_ThreadID() in this header, so the following `#define` sets `SDL_ThreadID()`
* to an invalid value, forcing you to consciously adjust it to your needs, i.e.
* replacing it with `SDL_GetCurrentThreadID()` or `0` depending on your intentions.
*
* Note that using `SDL_ThreadID x = ...;` is unaffected, only "SDL_ThreadID" followed
* by "(" will lead to a compile error.
*
* Unfortunately, there are some cases where this breaks legit SDL3 code that
* actually means to use `SDL_ThreadID(...`, like:
* - Definitions of function pointers with SDL_ThreadID as return type, like
* `typedef SDL_ThreadID (SDLCALL *SDL_DYNAPIFN_SDL_GetCurrentThreadID) (void);`
* - C++ casts like `SDL_ThreadID tid = SDL_ThreadID(id);`
*
* For C++ casts, just replace `SDL_ThreadID(id)` with either a C-style cast
* ( `(SDL_ThreadID)id` ) or one of the other C++ cast types like
* `static_cast<SDL_ThreadID>(id)`.
*
* For the (hopefully rare) case of function pointers returning SDL_ThreadID,
* or any other legit case where `SDL_ThreadID` is followed by `(` that can't be
* easily avoided by using slightly different syntax, you can `#undef SDL_ThreadID`
* before your code using it to get rid of this define causing the compiler error.
*
* If you're using `SDL_ENABLE_OLD_NAMES` to support both SDL2 and SDL3 with
* the same code, consider adding:
*
* ```c
* #if SDL_MAJOR_VERSION == 2
* #define SDL_GetCurrentThreadID() SDL_ThreadID()
* #endif
* ```
*
* and using `SDL_GetCurrentThreadID()` in your code, even if you otherwise use
* the SDL2 names.
*
* The gain of catching the bugs caused by accidentally using `SDL_ThreadID()`
* when `SDL_GetCurrentThreadID()` was intended hopefully outweight the annoyances
* caused by this in some rare cases.
*/
#define SDL_ThreadID() SDL_ThreadID_renamed_SDL_GetCurrentThreadID
#endif /* SDL_oldnames_h_ */