Re: conformsToProtocol over DO

[Richard Frith-Macdonald]

On 19 Apr 2004, at 07:45, Philip Mötteli wrote:

> Hi
>
>
> I have a heavy crash, that I do not understand why:
> I send from a client program the following message:
>
>         [aUser conformsToProtocol:@protocol(UserProtocol)];
>
>
> This makes my server crash in
>
>         #2 in +[NSObject conformsToProtocol:aProtocol] at NSObject.m:1286
>
>
> where the protocol list gives something probably wrong:
>
>         (gdb) print *aProtocol
> 	$3 = {isa = 0x4022c680, protocol_name = 0x0, protocol_list = 
> 0c8165c78, instance_methods = 0x0, class_methods = 0x1}
>         (gdb) print *aProtocol->protocol_list
>         $2 = {next = 0x40228580, count = 257, list = {0x0}}
>
>
> Count is surely wrong and the protocol name of the first protocol too.
> So the program crashes with somewhere in libc:
>
>         #0 in strcmp () from /lib/i686/libc.so.6
>         #1 in -[Protocol conformsTo:] () from /usr/lib/libobjc.so.1
>         #2 in +[NSObject conformsToProtocol:] at NSObject.m:1286
>         #3 in -[NSObject conformsToProtocol:] at NSObject.m:1309
>         #4 in __builtin_avcall () from /usr/lib/libavcall.so.0
>         #5 in GSFFCallInvokeWithTargetAndImp at GSFFCallInvocation.m:638
>         #6 in callframe_do_call at callframe.m:442
> 	#7 in -[NSConnection(Private) _service:forwardForProxy:] at 
> NSConnection.m:2390
> 	#8 in -[NSConnection(Private) handlePortMessage:] at 
> NSConnection.m:2149
>         #9 in -[NSSocketPort handlePortMessage:] at NSSocketPort.m:1712
> 	#10 in -[GSTcpHandle receivedEvent:type:extra:forMode:] at 
> NSSocketPort.m:1021
> 	#11 in -[NSSocketPort receivedEvent:type:extra:forMode:] at 
> NSSocketPort.m:1864
>         #12 in -[GSRunLoopCtxt pollUntil:within:] at NSRunLoop.m:835
>         #13 in -[NSRunLoop acceptInputForMode:beforeDate:] at NSRunLoop.m:1960
>         #14 in -[NSRunLoop runMode:beforeDate:] at NSRunLoop.m:2030
>         #15 in -[NSRunLoop runUntilDate:] at NSRunLoop.m:2051
>         #16 in -[NSRunLoop run] at NSRunLoop.m:2039
>         #17 in main (argc=1, argv=0xbffff434) at main.m:62
>
>
> if I replace the call to -conformsToProtocol: with
>
>         [aUser respondsToSelector:@selector(loginName)]
>
> it works.
>
>
> Does anybody have an idea, why this crashes? Or why perhaps the 
> protocol is not correctly sent over the wire?

Looking at the source of  Protocol in the runtime, it accesses ivars of 
the other protocol object directly ... so it can't work via a proxy to 
the remote system ... but the Protocol objects have no coding/decoding 
to be sent over the wire and no clean interface which could allow you 
to implement such coding.
I've hacked in a solution in NSDistantObject.m which is still rather 
nasty ... but at least it only
accesses one private ivar.