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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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<h2>Selective Symbol Loading</h2> |
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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>SymbolLoadException</tt> when a shared library symbol |
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cannot be found. In order to allow the client to bypass this default behavior, |
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the <a href="util.html">DerelictUtil</a> package provides a callback mechanism |
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that allows you to selectively throw exceptions when symbols are missing. |
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<h3>MissingSymbolCallback</h3> |
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The <tt>derelict.util.exception</tt> module defines a type <tt>MissingSymbolCallback</tt>. |
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The type definition looks like this: |
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<pre> |
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alias bool function(string libName, string symbolName) MissingSymbolCallback; |
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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>MissingSymbolCallback</tt> |
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has been set. If not, a <tt>SymbolLoadException</tt> is thrown. But, if a callback |
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has been set, then the callback is called with the name of the library |
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and the name of the missing symbol as arguments. The callback should return <tt>true</tt> if |
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loading should continue and <tt>false</tt> if a <tt>SymbolLoadException</tt> should be thrown. |
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Callbacks can be set using the function <tt>Derelict_SetMissingSymbolCallback</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; // MissingSymbolCallback interface declared here |
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import derelict.sdl.sdl; |
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import std.string; |
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bool SDLMissingSymbolCallback(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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It is important to note that the callback must be set before loading the library |
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you are interested in filtering. You can implement one callback for multiple libraries if you like, |
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and filter them based upon the <tt>libName</tt> parameter. Or you can define multiple callbacks to |
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handle one library each - as long as you call <tt>Derelict_SetMissingSymbolCallback</tt> with the |
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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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