From 56355f3ffe8b681799ab2f1a0d03bb3bfc45f41d Mon Sep 17 00:00:00 2001 From: Gabriel Jacobo Date: Wed, 15 Jan 2014 12:29:17 -0300 Subject: [PATCH] Adds README-dynapi.txt based on Ryan's G+ post (lightly edited) Original post: https://plus.google.com/103391075724026391227/posts/TB8UfnDYu4U --- README-dynapi.txt | 130 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 130 insertions(+) create mode 100644 README-dynapi.txt diff --git a/README-dynapi.txt b/README-dynapi.txt new file mode 100644 index 000000000..da52f3a32 --- /dev/null +++ b/README-dynapi.txt @@ -0,0 +1,130 @@ +================================================================================ +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.