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digitalmars.D.learn - Dynamic Loading of C DLLs and How to Use Them

braddr from #d asked me to post this here, so here we go :)

This has probably been posted before, but it's useful to keep in an 
accessible place. When trying to cast a function pointer to an external 
C version of one, it requires this really awkward trick.

Placing the alias outside of a class at the module level inside an 
extern (C) will make an alias of a C function pointer. You can then use 
this alias to convert the function pointers. Here is an example:

-- Boilerplate types --

extern (C)
   alias void function() CFunc;
   void function() testVar;

alias void function() NotCFunc;

-- Usage example --

// This does NOT work and gripes about unable to convert to C
testVar = cast(NotCFunc)GetSomeCFuncPointer();

// This DOES work
testVar = cast(CFunc)GetSomeCFuncPointer();

-- End of Example --

 From my observations the alias command will capture the external state 
as part of the alias, which means this is the only way to cast to C 
function pointers at this time.

-Joshua Cearley
BlackCrystal Software
Mar 22 2006