Re: NSInvocation return value location

[David Chisnall]

I've looked at GSFFCallInvocation and I can't work out why changing  
the value of _retval does not alter where it stores the return value.   
av_start_ptr appears to be called with _retval as the argument, but  
the result is written both to the location specified there and on the  
original stack location.

Any hints?

David

On 30 Nov 2008, at 02:36, David Chisnall wrote:

> 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
>
>
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