docs/README-dynapi.md
author Philipp Wiesemann <philipp.wiesemann@arcor.de>
Sun, 15 Mar 2015 19:25:10 +0100
changeset 9383 62164ad0b7d5
parent 9025 d09d4b578e77
child 9754 fe6acfd4652c
permissions -rw-r--r--
Updated name of assert type in test program.
     1 Dynamic API
     2 ================================================================================
     3 Originally posted by Ryan at https://plus.google.com/103391075724026391227/posts/TB8UfnDYu4U
     4 
     5 Background:
     6 
     7 - The Steam Runtime has (at least in theory) a really kick-ass build of SDL2, 
     8   but developers are shipping their own SDL2 with individual Steam games. 
     9   These games might stop getting updates, but a newer SDL2 might be needed later. 
    10   Certainly we'll always be fixing bugs in SDL, even if a new video target isn't 
    11   ever needed, and these fixes won't make it to a game shipping its own SDL.
    12 - Even if we replace the SDL2 in those games with a compatible one, that is to 
    13   say, edit a developer's Steam depot (yuck!), there are developers that are 
    14   statically linking SDL2 that we can't do this for. We can't even force the 
    15   dynamic loader to ignore their SDL2 in this case, of course.
    16 - If you don't ship an SDL2 with the game in some form, people that disabled the
    17   Steam Runtime, or just tried to run the game from the command line instead of 
    18   Steam might find themselves unable to run the game, due to a missing dependency.
    19 - If you want to ship on non-Steam platforms like GOG or Humble Bundle, or target
    20   generic Linux boxes that may or may not have SDL2 installed, you have to ship 
    21   the library or risk a total failure to launch. So now, you might have to have 
    22   a non-Steam build plus a Steam build (that is, one with and one without SDL2 
    23   included), which is inconvenient if you could have had one universal build 
    24   that works everywhere.
    25 - We like the zlib license, but the biggest complaint from the open source 
    26   community about the license change is the static linking. The LGPL forced this 
    27   as a legal, not technical issue, but zlib doesn't care. Even those that aren't
    28   concerned about the GNU freedoms found themselves solving the same problems: 
    29   swapping in a newer SDL to an older game often times can save the day. 
    30   Static linking stops this dead.
    31 
    32 So here's what we did:
    33 
    34 SDL now has, internally, a table of function pointers. So, this is what SDL_Init
    35 now looks like:
    36 
    37     UInt32 SDL_Init(Uint32 flags)
    38     {
    39         return jump_table.SDL_Init(flags);
    40     }
    41 
    42 Except that is all done with a bunch of macro magic so we don't have to maintain
    43 every one of these.
    44 
    45 What is jump_table.SDL_init()? Eventually, that's a function pointer of the real
    46 SDL_Init() that you've been calling all this time. But at startup, it looks more 
    47 like this:
    48 
    49     Uint32 SDL_Init_DEFAULT(Uint32 flags)
    50     {
    51         SDL_InitDynamicAPI();
    52         return jump_table.SDL_Init(flags);
    53     }
    54 
    55 SDL_InitDynamicAPI() fills in jump_table with all the actual SDL function 
    56 pointers, which means that this _DEFAULT function never gets called again. 
    57 First call to any SDL function sets the whole thing up.
    58 
    59 So you might be asking, what was the value in that? Isn't this what the operating
    60 system's dynamic loader was supposed to do for us? Yes, but now we've got this 
    61 level of indirection, we can do things like this:
    62 
    63     export SDL_DYNAMIC_API=/my/actual/libSDL-2.0.so.0
    64     ./MyGameThatIsStaticallyLinkedToSDL2
    65 
    66 And now, this game that is staticallly linked to SDL, can still be overridden 
    67 with a newer, or better, SDL. The statically linked one will only be used as 
    68 far as calling into the jump table in this case. But in cases where no override
    69 is desired, the statically linked version will provide its own jump table, 
    70 and everyone is happy.
    71 
    72 So now:
    73 - Developers can statically link SDL, and users can still replace it. 
    74   (We'd still rather you ship a shared library, though!)
    75 - Developers can ship an SDL with their game, Valve can override it for, say, 
    76   new features on SteamOS, or distros can override it for their own needs, 
    77   but it'll also just work in the default case.
    78 - Developers can ship the same package to everyone (Humble Bundle, GOG, etc), 
    79   and it'll do the right thing.
    80 - End users (and Valve) can update a game's SDL in almost any case, 
    81   to keep abandoned games running on newer platforms.
    82 - Everyone develops with SDL exactly as they have been doing all along. 
    83   Same headers, same ABI. Just get the latest version to enable this magic.
    84 
    85 
    86 A little more about SDL_InitDynamicAPI():
    87 
    88 Internally, InitAPI does some locking to make sure everything waits until a 
    89 single thread initializes everything (although even SDL_CreateThread() goes 
    90 through here before spinning a thread, too), and then decides if it should use
    91 an external SDL library. If not, it sets up the jump table using the current 
    92 SDL's function pointers (which might be statically linked into a program, or in
    93 a shared library of its own). If so, it loads that library and looks for and 
    94 calls a single function:
    95 
    96     SInt32 SDL_DYNAPI_entry(Uint32 version, void *table, Uint32 tablesize);
    97 
    98 That function takes a version number (more on that in a moment), the address of
    99 the jump table, and the size, in bytes, of the table. 
   100 Now, we've got policy here: this table's layout never changes; new stuff gets 
   101 added to the end. Therefore SDL_DYNAPI_entry() knows that it can provide all 
   102 the needed functions if tablesize <= sizeof its own jump table. If tablesize is
   103 bigger (say, SDL 2.0.4 is trying to load SDL 2.0.3), then we know to abort, but
   104 if it's smaller, we know we can provide the entire API that the caller needs.
   105 
   106 The version variable is a failsafe switch. 
   107 Right now it's always 1. This number changes when there are major API changes 
   108 (so we know if the tablesize might be smaller, or entries in it have changed). 
   109 Right now SDL_DYNAPI_entry gives up if the version doesn't match, but it's not 
   110 inconceivable to have a small dispatch library that only supplies this one 
   111 function and loads different, otherwise-incompatible SDL libraries and has the
   112 right one initialize the jump table based on the version. For something that 
   113 must generically catch lots of different versions of SDL over time, like the 
   114 Steam Client, this isn't a bad option.
   115 
   116 Finally, I'm sure some people are reading this and thinking,
   117 "I don't want that overhead in my project!"  
   118 To which I would point out that the extra function call through the jump table 
   119 probably wouldn't even show up in a profile, but lucky you: this can all be 
   120 disabled. You can build SDL without this if you absolutely must, but we would 
   121 encourage you not to do that. However, on heavily locked down platforms like 
   122 iOS, or maybe when debugging,  it makes sense to disable it. The way this is 
   123 designed in SDL, you just have to change one #define, and the entire system 
   124 vaporizes out, and SDL functions exactly like it always did. Most of it is 
   125 macro magic, so the system is contained to one C file and a few headers. 
   126 However, this is on by default and you have to edit a header file to turn it 
   127 off. Our hopes is that if we make it easy to disable, but not too easy, 
   128 everyone will ultimately be able to get what they want, but we've gently 
   129 nudged everyone towards what we think is the best solution.