docs/README-dynapi.md
changeset 9025 d09d4b578e77
parent 9023 276802355854
child 9754 fe6acfd4652c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/docs/README-dynapi.md	Tue Jul 29 08:04:15 2014 -0700
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+Dynamic API
+================================================================================
+Originally posted by Ryan at https://plus.google.com/103391075724026391227/posts/TB8UfnDYu4U
+
+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:
+
+    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:
+
+    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:
+
+    export SDL_DYNAMIC_API=/my/actual/libSDL-2.0.so.0
+    ./MyGameThatIsStaticallyLinkedToSDL2
+
+And now, this game that is staticallly 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:
+
+    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.