Re: linking tool to use a framework : obj_send_msg error

[David Chisnall]

On 12 Apr 2009, at 22:23, Thomas Kupper wrote:

> Next I created a very minimalistic tool application which uses the  
> log4cocoa framework (of course I first installed it into the USER  
> domain). Now the linker throws an error message
> "Linking tool ocHello ..."
> "/home/thomas/GNUstep/Library/Libraries/liblog4cocoa.so: undefined  
> reference to 'objc_msgSend'".
> "collect2: ld returned 1 exit status"

There is probably some 'optimisation' in the framework calling the  
Apple runtime message send function directly (the quotes because  
anything doing I/O like this is likely to gain an imperceptible  
performance increase from calling the runtime functions directly  
rather than using -methodForSelector:).  Try grepping the source code  
for objc_msgSend() and, in files that contain it, add this macro:

#define objc_msgSend(theReceiver, theSelector, ...)  \
	objc_msg_lookup(theReceiver, theSelector)(theReceiver, theSelector,  
## __VA_LIST__)

> How can that happen if the framework compiles/links fine. And the  
> tool compiles/links fine, too without using the framework.

Libraries are not checked for unresolved symbols when they are  
linked.  This allows you to create a library which depends on a  
function that is declared in an application, providing a fast  
(although very fragile, unsafe, and stupid) way of implementing  
callbacks, but is mainly intended for static libraries which are  
intended to be combined with others to provide a full implementation  
of an API.

> Can someone give me a hint how I can track down that error?

Look for objc_msgSend() in the log4cocoa framework source.

David