Am 30.06.2012 03:25 schrieb "kyan" <alfasud...@gmail.com>: > A pascal open array of const which is the closest concept to a C > ellipsis may have similar semantics but its binary representation on > the parameter stack is completely different. First of all, since it > has to work in all calling conventions and not just cdecl the high > bound of passed array elements is pushed in the stack after a pointer > to an array of TVarRec structures that each contains a type attribute > and either a value or a pointer to the value of the parameter. So the > open array doesn't need to be the last parameter like the C ellipsis > and there can be more than one in a routine signature. So your > declaration of purple_prpl_got_user_status > > procedure purple_prpl_got_user_status(account: PPurpleAccount; > name_, status_id: PChar; > par3: array of const); > > is equivalent to: > > procedure purple_prpl_got_user_status(account: PPurpleAccount; > name_, status_id: PChar; > par3: PVarRecArray; par3HighBound: Integer); > > So the caller pushes only a pointer to a TVarRec array and an integer > while the function expects a continuous "stream" of parameters. If you > pass [] the compiler generates code to push a nil pointer (zero > PtrInt) and a high bound of -1. If the C function can handle these > values on the stack it *sort of* works by accident, but it will > probably fail if it tries to read past the -1 high bound for a > specific value of status_id that requires more variable parameters.
Your explanation is very nice and mostly correct, but in case of external cdecl functions in FPC the "varargs" modifier and a "array of const" parameter imply the same C compatible parameter passing in the compiler. See also here: http://freepascal.org/docs-html/ref/refsu77.html#x180-19000014.9.17 The only difference is when calling the functions. E.g.: procedure a(args: array of const); cdecl; external; procedure b; cdecl; varargs; external; begin a([1, 'foo', somevar]); b(1, 'foo', somevar); end. Regards, Sven
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