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+# Dynamic API

+

+Originally posted on Ryan's Google+ account.

+

+Background:

+

+- The Steam Runtime has (at least in theory) a really kick-ass build of SDL2,

+  but developers are shipping their own SDL2 with individual Steam games.

+  These games might stop getting updates, but a newer SDL2 might be needed later.

+  Certainly we'll always be fixing bugs in SDL, even if a new video target isn't

+  ever needed, and these fixes won't make it to a game shipping its own SDL.

+- Even if we replace the SDL2 in those games with a compatible one, that is to

+  say, edit a developer's Steam depot (yuck!), there are developers that are

+  statically linking SDL2 that we can't do this for. We can't even force the

+  dynamic loader to ignore their SDL2 in this case, of course.

+- If you don't ship an SDL2 with the game in some form, people that disabled the

+  Steam Runtime, or just tried to run the game from the command line instead of

+  Steam might find themselves unable to run the game, due to a missing dependency.

+- If you want to ship on non-Steam platforms like GOG or Humble Bundle, or target

+  generic Linux boxes that may or may not have SDL2 installed, you have to ship

+  the library or risk a total failure to launch. So now, you might have to have

+  a non-Steam build plus a Steam build (that is, one with and one without SDL2

+  included), which is inconvenient if you could have had one universal build

+  that works everywhere.

+- We like the zlib license, but the biggest complaint from the open source

+  community about the license change is the static linking. The LGPL forced this

+  as a legal, not technical issue, but zlib doesn't care. Even those that aren't

+  concerned about the GNU freedoms found themselves solving the same problems:

+  swapping in a newer SDL to an older game often times can save the day.

+  Static linking stops this dead.

+

+So here's what we did:

+

+SDL now has, internally, a table of function pointers. So, this is what SDL_Init

+now looks like:

+

+```c

+Uint32 SDL_Init(Uint32 flags)

+{

+    return jump_table.SDL_Init(flags);

+}

+```

+

+Except that is all done with a bunch of macro magic so we don't have to maintain

+every one of these.

+

+What is jump_table.SDL_init()? Eventually, that's a function pointer of the real

+SDL_Init() that you've been calling all this time. But at startup, it looks more

+like this:

+

+```c

+Uint32 SDL_Init_DEFAULT(Uint32 flags)

+{

+    SDL_InitDynamicAPI();

+    return jump_table.SDL_Init(flags);

+}

+```

+

+SDL_InitDynamicAPI() fills in jump_table with all the actual SDL function

+pointers, which means that this `_DEFAULT` function never gets called again.

+First call to any SDL function sets the whole thing up.

+

+So you might be asking, what was the value in that? Isn't this what the operating

+system's dynamic loader was supposed to do for us? Yes, but now we've got this

+level of indirection, we can do things like this:

+

+```bash

+export SDL_DYNAMIC_API=/my/actual/libSDL-2.0.so.0

+./MyGameThatIsStaticallyLinkedToSDL2

+```

+

+And now, this game that is statically linked to SDL, can still be overridden

+with a newer, or better, SDL. The statically linked one will only be used as

+far as calling into the jump table in this case. But in cases where no override

+is desired, the statically linked version will provide its own jump table,

+and everyone is happy.

+

+So now:

+- Developers can statically link SDL, and users can still replace it.

+  (We'd still rather you ship a shared library, though!)

+- Developers can ship an SDL with their game, Valve can override it for, say,

+  new features on SteamOS, or distros can override it for their own needs,

+  but it'll also just work in the default case.

+- Developers can ship the same package to everyone (Humble Bundle, GOG, etc),

+  and it'll do the right thing.

+- End users (and Valve) can update a game's SDL in almost any case,

+  to keep abandoned games running on newer platforms.

+- Everyone develops with SDL exactly as they have been doing all along.

+  Same headers, same ABI. Just get the latest version to enable this magic.

+

+

+A little more about SDL_InitDynamicAPI():

+

+Internally, InitAPI does some locking to make sure everything waits until a

+single thread initializes everything (although even SDL_CreateThread() goes

+through here before spinning a thread, too), and then decides if it should use

+an external SDL library. If not, it sets up the jump table using the current

+SDL's function pointers (which might be statically linked into a program, or in

+a shared library of its own). If so, it loads that library and looks for and

+calls a single function:

+

+```c

+Sint32 SDL_DYNAPI_entry(Uint32 version, void *table, Uint32 tablesize);

+```

+

+That function takes a version number (more on that in a moment), the address of

+the jump table, and the size, in bytes, of the table.

+Now, we've got policy here: this table's layout never changes; new stuff gets

+added to the end. Therefore SDL_DYNAPI_entry() knows that it can provide all

+the needed functions if tablesize <= sizeof its own jump table. If tablesize is

+bigger (say, SDL 2.0.4 is trying to load SDL 2.0.3), then we know to abort, but

+if it's smaller, we know we can provide the entire API that the caller needs.

+

+The version variable is a failsafe switch.

+Right now it's always 1. This number changes when there are major API changes

+(so we know if the tablesize might be smaller, or entries in it have changed).

+Right now SDL_DYNAPI_entry gives up if the version doesn't match, but it's not

+inconceivable to have a small dispatch library that only supplies this one

+function and loads different, otherwise-incompatible SDL libraries and has the

+right one initialize the jump table based on the version. For something that

+must generically catch lots of different versions of SDL over time, like the

+Steam Client, this isn't a bad option.

+

+Finally, I'm sure some people are reading this and thinking,

+"I don't want that overhead in my project!"

+

+To which I would point out that the extra function call through the jump table

+probably wouldn't even show up in a profile, but lucky you: this can all be

+disabled. You can build SDL without this if you absolutely must, but we would

+encourage you not to do that. However, on heavily locked down platforms like

+iOS, or maybe when debugging, it makes sense to disable it. The way this is

+designed in SDL, you just have to change one #define, and the entire system

+vaporizes out, and SDL functions exactly like it always did. Most of it is

+macro magic, so the system is contained to one C file and a few headers.

+However, this is on by default and you have to edit a header file to turn it

+off. Our hopes is that if we make it easy to disable, but not too easy,

+everyone will ultimately be able to get what they want, but we've gently

+nudged everyone towards what we think is the best solution.