[Adium-devl] GPL and Objective C

[Ofri Wolfus]

Hi everyone,

A discussion with David in IRC has lead me to talk with a friend of  
mine (who is a lawyer) about the nature of objc code released under  
GPL. As you probably know, a message to an object is translated by  
the compiler to a call to one of the objc_msgSend() functions.  
Therefor, you never get to call any 'real' function from your code  
and you only indirectly call GPL functions.
Well, GPL code forbids you from linking to it unless your code is GPL  
as well, but what happens if the GPL code lives in a plugin which is  
dynamically loaded? This is what GPL has to say about it (taken from  
http://www.gnu.org/licenses/gpl-faq.html#GPLAndPlugins):

"It depends on how the program invokes its plug-ins. If the program  
uses fork and exec to invoke plug-ins, then the plug-ins are separate  
programs, so the license of the plug-in makes no requirements about  
the main program.
If the program dynamically links plug-ins, and they make function  
calls to each other and share data structures, we believe they form a  
single program, which must be treated as an extension of both the  
main program and the plug-ins. In order to use the GPL-covered plug- 
ins, the main program must be released under the GPL or a GPL- 
compatible free software license, and that the terms of the GPL must  
be followed when the main program is distributed for use with these  
plug-ins.
If the program dynamically links plug-ins, but the communication  
between them is limited to invoking the `main' function of the plug- 
in with some options and waiting for it to return, that is a  
borderline case."

But message sending is not a function call... Therefor, a message  
being sent to a GPL class, does not force the sender to be GPL as  
well. As long as your code does not set instance variables of a GPL  
class (directly - accessors are fine), use IMP caching or use the  
@defs() directive, your code DOES NOT have to be released under GPL  
license.

But what about the classes themselves? GPL specifically says that one  
can not use GPL data structures in a non-GPL code. Well... it doesn't  
bother us at all since classes are translated by the compiler to the  
objc_class struct which has no effect on us. The same goes with  
instances of classes which are translated to pointers to an  
objc_object struct.

Quite revolutionary ha? ;-) Using plugins, commercial apps can take  
advantage of ANY GPL code that can be wrapped in a cocoa class. Not  
so good... But for us it also means some good stuff. It means that  
Adium and ChatKit can use protocol libraries which are not GPL. :D

- Ofri