NSInvocation return value location

[David Chisnall]

Hi,

I have some code which transfers an NSInvocation to another thread to  
execute.  Before doing this, it sets the return value to a proxy.  I  
now have the problem on GNUstep that when I fire the invocation in the  
second thread that it writes the result to the stack on the original  
thread.

OS X does something with NSInvocations that deals with the special  
case that the function constructing the NSInvocation returns before  
the NSInvocation is fired by having the function that calls  
forwardInvocation: block until the invocation has fired once and then,  
when the function returns, setting the invocation to store its result  
internally in a buffer that can be accessed with getReturnValue:.

With GNUstep it is possible to emulate this behaviour using libffi but  
not ffcall.  The _storeRetval method on NSInvocation appears to work  
correctly with FFI, ensuring that the return value is stored in the  
invocation.  With ffcall, this method is not implemented, and setting  
_retval manually results in the return value being stored in both the  
specified location and in the original location.

David