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<html lang="en"> |
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<head> |
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<title>Selective Symbol Loading</title> |
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<link rel="stylesheet" type="text/css" href="styles.css"> |
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</head> |
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<body> |
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<hr> |
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<hr> |
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<h2 align="center">Selective Symbol Loading</h2> |
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<hr> |
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<hr> |
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<h3>Introduction</h3> |
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There may be times when you want to provide backwards compatibility with an |
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older version of a library, or when you know that a particular build of a library |
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you are using is missing some functions that a Derelict package loads. Derelict, |
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by default, throws a <tt>SharedLibProcLoadException</tt> when a shared library symbol |
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cannot be found, and ceases loading the library. In order to allow the client to |
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bypass this default behavior, the <a href="util.html">DerelictUtil</a> package |
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provides a callback mechanism that allows you to selectively throw exceptions |
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when symbols are missing. |
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<h3>MissingProcCallback</h3> |
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The <tt>derelict.util.exception</tt> module defines a type <tt>MissingProcCallback</tt>. |
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The type definition looks like this: |
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<pre> |
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typedef bool function(char[] libName, char[] procName) MissingProcCallback; |
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</pre> |
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<p> |
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When a symbol fails to load, Derelict first checks if a <tt>MissingProcCallback</tt> |
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has been set. If it has, then the callback is called with the name of the library |
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and the name of the missing symbol as arguments. If a callback has not been set, |
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then a <tt>SharedLibProcLoadException</tt> is thrown instead. Callbacks can be |
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set using the function <tt>Derelict_SetMissingProcCallback</tt>. |
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</p> |
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<p> |
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Following is a complete example of using this feature. Older versions of SDL |
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did not include functions to test for CPU capabilities. |
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<a href="sdl.html">DerelictSDL</a> always attempts to load those functions since |
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they are present in the latest versions. It is possible to load older versions |
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of the SDL shared library by ignoring the CPU related functions. This example |
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demonstrates how to do that. |
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</p> |
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<pre> |
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import derelict.util.exception; // MissingProcCallback interface declared here |
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import derelict.sdl.sdl; |
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import std.string; |
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bool myMissingProcCallback(char[] libName, char[] procName) |
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{ |
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// there are 8 functions in SDL's CPU interface - test for them all. |
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// If the procName matches any one of them, return true to ignore the missing |
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// function. |
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if( procName.cmp("SDL_HasRDTSC") == 0 || |
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procName.cmp("SDL_HasMMX") == 0 || |
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procName.cmp("SDL_HasMMXExt") == 0 || |
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procName.cmp("SDL_Has3DNow") == 0 || |
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procName.cmp("SDL_Has3DNowExt") == 0 || |
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procName.cmp("SDL_HasSSE") == 0 || |
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procName.cmp("SDL_HasSSE2") == 0 || |
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procName.cmp("SDL_HasAltiVec") == 0) |
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return true; // ignore the error and throw no exception |
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// a function other than one of those above failed to load - return false |
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// to indicate that an exception should be thrown. |
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return false; |
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} |
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void main() |
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{ |
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// the callback must be set before loading the library(ies) you want the callback to handle |
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Derelict_SetMissingProcCallback(&myMissingProcCallback); |
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DerelictSDL.load(); |
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} |
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</pre> |
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<p> |
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As the example demonstrates, the callback should return <tt>true</tt> if Derelict should |
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ignore the missing symbol and continue loading the library. It should return <tt>false</tt> if an |
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exception should be thrown. It is also important to note that the callback must |
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be set before loading the library you are interested in filtering. You can implement |
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one callback for multiple libraries if you like, and filter them based upon the |
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<tt>libName</tt> parameter. Or you can define multiple callbacks to handle one |
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library each - as long as you call <tt>Derelict_SetMissingProcCallback</tt> with |
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the appropriate callback argument before loading each library. |
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</p> |
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<p> |
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<span class="important">This is an optional feature and is not required to use |
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Derelict</span>. It is provided to give an even greater degree of flexibility |
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and control to those who need it. |
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</p> |
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</body> |
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</html> |
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