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From: eckel@erik.ircam.fr (Gerhard Eckel)
Newsgroups: comp.sys.next
Subject: what Obj-C methods return
Keywords: Objective-C
Message-ID: <1062@ircam.ircam.fr>
Date: 3 May 90 17:20:43 GMT
Sender: news@ircam.ircam.fr
Reply-To: eckel@ircam.UUCP (Gerhard Eckel)
Distribution: comp.sys.next
Organization: l'Institut de Recherche et Coordination Acoustique-Musique
Lines: 41


	I try to write a 'send' function for a SMALL lisp interpreter that 
sends messages to Obj-C objects and receives the return values from the
corresponding methods (this is similar to the 'send' function of the objc 
package in Common Lisp on the NeXT). I try to use objc_msgSend() which returns
an id. I did some assembler hacking to get the other 4 bytes (asm("movel d1,
something")). I found that Obj-C doesn't return structures correctly that are
bigger than 8 bytes. C functions return such structures over the stack (it
seems that the compiler uses the register a1 to pass the address - data
smaller than 8 bytes is returned in the registers d0 and d1 as far as I could
see). It seems that it is a bug since the methods try to copy the data using
register a1. But where does it end up (I hate 'Bus error' and 'Segmentation
fault')?
	
Consider the following fragment:
	
	typedef struct { int a, b, c; } threeInts;
	
	@interface SomeClass : SomeSuperClass
	{ threeInts someInts; }
	- (threeInts) someMethod;
	@end
	
	@implementation SomeClass
	- (threeInts) someMethod { return someInts; }
	@end
	
	id instance = [SomeClass new];
	
	// the following line doesn't work
	// (i.e. the data is not copied into r)
	
	threeInts result = [instance someMethod];
	
	// it works for twoInts (struct { int a, b; })
	
Any idea? Is this how it should be? Anything in the doc that I missed? Anybody else who tried to do a similar thing?

Gerhard Eckel
IRCAM, Paris
eckel@ircam.fr