[Adium-devl] GPL and Objective C

Adam Iser adam at mutablecopy.net
Sun Mar 5 22:16:38 UTC 2006


I just wrote a block of assembly which JMPs into GPL code (A JMP is not 
a function call! HA! HA!).  All your GPL code are belong to me.

- Adam

Ofri Wolfus wrote:

>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
>
>
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