Query about call-process

[Narendra Joshi]

Hi,

The document string for `call-process' says that
>           You can’t directly specify a buffer to put the error output
>           in; that is too difficult to implement.  But you can achieve
>           this result by sending the error output to a temporary file
>           and then inserting the file into a buffer when the subprocess
>           finishes.

I would like to understand what makes doing this difficult? 

Thanks,
-- 
Narendra Joshi