docs/README-dynapi.md
author Sam Lantinga <slouken@libsdl.org>
Tue, 23 Apr 2019 07:59:31 -0700
changeset 12714 9b7633bd0aa0
parent 11720 9cbb45a5874f
permissions -rw-r--r--
Use _Exit() when available
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Dynamic API
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================================================================================
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Originally posted by Ryan at:
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  https://plus.google.com/103391075724026391227/posts/TB8UfnDYu4U
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Background:
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- The Steam Runtime has (at least in theory) a really kick-ass build of SDL2, 
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  but developers are shipping their own SDL2 with individual Steam games. 
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  These games might stop getting updates, but a newer SDL2 might be needed later. 
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  Certainly we'll always be fixing bugs in SDL, even if a new video target isn't 
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  ever needed, and these fixes won't make it to a game shipping its own SDL.
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- Even if we replace the SDL2 in those games with a compatible one, that is to 
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  say, edit a developer's Steam depot (yuck!), there are developers that are 
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  statically linking SDL2 that we can't do this for. We can't even force the 
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  dynamic loader to ignore their SDL2 in this case, of course.
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- If you don't ship an SDL2 with the game in some form, people that disabled the
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  Steam Runtime, or just tried to run the game from the command line instead of 
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  Steam might find themselves unable to run the game, due to a missing dependency.
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- If you want to ship on non-Steam platforms like GOG or Humble Bundle, or target
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  generic Linux boxes that may or may not have SDL2 installed, you have to ship 
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  the library or risk a total failure to launch. So now, you might have to have 
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  a non-Steam build plus a Steam build (that is, one with and one without SDL2 
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  included), which is inconvenient if you could have had one universal build 
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  that works everywhere.
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- We like the zlib license, but the biggest complaint from the open source 
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  community about the license change is the static linking. The LGPL forced this 
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  as a legal, not technical issue, but zlib doesn't care. Even those that aren't
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  concerned about the GNU freedoms found themselves solving the same problems: 
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  swapping in a newer SDL to an older game often times can save the day. 
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  Static linking stops this dead.
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So here's what we did:
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SDL now has, internally, a table of function pointers. So, this is what SDL_Init
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now looks like:
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    UInt32 SDL_Init(Uint32 flags)
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    {
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        return jump_table.SDL_Init(flags);
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    }
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Except that is all done with a bunch of macro magic so we don't have to maintain
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every one of these.
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What is jump_table.SDL_init()? Eventually, that's a function pointer of the real
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SDL_Init() that you've been calling all this time. But at startup, it looks more 
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like this:
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    Uint32 SDL_Init_DEFAULT(Uint32 flags)
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    {
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        SDL_InitDynamicAPI();
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        return jump_table.SDL_Init(flags);
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    }
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SDL_InitDynamicAPI() fills in jump_table with all the actual SDL function 
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pointers, which means that this _DEFAULT function never gets called again. 
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First call to any SDL function sets the whole thing up.
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So you might be asking, what was the value in that? Isn't this what the operating
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system's dynamic loader was supposed to do for us? Yes, but now we've got this 
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level of indirection, we can do things like this:
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    export SDL_DYNAMIC_API=/my/actual/libSDL-2.0.so.0
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    ./MyGameThatIsStaticallyLinkedToSDL2
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And now, this game that is statically linked to SDL, can still be overridden 
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with a newer, or better, SDL. The statically linked one will only be used as 
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far as calling into the jump table in this case. But in cases where no override
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is desired, the statically linked version will provide its own jump table, 
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and everyone is happy.
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So now:
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- Developers can statically link SDL, and users can still replace it. 
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  (We'd still rather you ship a shared library, though!)
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- Developers can ship an SDL with their game, Valve can override it for, say, 
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  new features on SteamOS, or distros can override it for their own needs, 
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  but it'll also just work in the default case.
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- Developers can ship the same package to everyone (Humble Bundle, GOG, etc), 
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  and it'll do the right thing.
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- End users (and Valve) can update a game's SDL in almost any case, 
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  to keep abandoned games running on newer platforms.
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- Everyone develops with SDL exactly as they have been doing all along. 
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  Same headers, same ABI. Just get the latest version to enable this magic.
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A little more about SDL_InitDynamicAPI():
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Internally, InitAPI does some locking to make sure everything waits until a 
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single thread initializes everything (although even SDL_CreateThread() goes 
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through here before spinning a thread, too), and then decides if it should use
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an external SDL library. If not, it sets up the jump table using the current 
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SDL's function pointers (which might be statically linked into a program, or in
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a shared library of its own). If so, it loads that library and looks for and 
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calls a single function:
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    SInt32 SDL_DYNAPI_entry(Uint32 version, void *table, Uint32 tablesize);
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That function takes a version number (more on that in a moment), the address of
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the jump table, and the size, in bytes, of the table. 
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Now, we've got policy here: this table's layout never changes; new stuff gets 
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added to the end. Therefore SDL_DYNAPI_entry() knows that it can provide all 
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the needed functions if tablesize <= sizeof its own jump table. If tablesize is
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bigger (say, SDL 2.0.4 is trying to load SDL 2.0.3), then we know to abort, but
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if it's smaller, we know we can provide the entire API that the caller needs.
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The version variable is a failsafe switch. 
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Right now it's always 1. This number changes when there are major API changes 
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(so we know if the tablesize might be smaller, or entries in it have changed). 
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Right now SDL_DYNAPI_entry gives up if the version doesn't match, but it's not 
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inconceivable to have a small dispatch library that only supplies this one 
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function and loads different, otherwise-incompatible SDL libraries and has the
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right one initialize the jump table based on the version. For something that 
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must generically catch lots of different versions of SDL over time, like the 
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Steam Client, this isn't a bad option.
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Finally, I'm sure some people are reading this and thinking,
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"I don't want that overhead in my project!"  
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To which I would point out that the extra function call through the jump table 
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probably wouldn't even show up in a profile, but lucky you: this can all be 
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disabled. You can build SDL without this if you absolutely must, but we would 
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encourage you not to do that. However, on heavily locked down platforms like 
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iOS, or maybe when debugging, it makes sense to disable it. The way this is
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designed in SDL, you just have to change one #define, and the entire system 
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vaporizes out, and SDL functions exactly like it always did. Most of it is 
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macro magic, so the system is contained to one C file and a few headers. 
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However, this is on by default and you have to edit a header file to turn it 
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off. Our hopes is that if we make it easy to disable, but not too easy, 
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everyone will ultimately be able to get what they want, but we've gently 
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nudged everyone towards what we think is the best solution.