Question regarding XGetImage in libx11

[Chuanlong Du]

 Thank you very much for the explanation, Alan!

    On Saturday, July 31, 2021, 9:36:07 AM PDT, Alan Coopersmith <alan.coopersmith at oracle.com> wrote:  
 
 On 7/30/21 10:49 PM, Chuanlong Du wrote:
> I was reading the source code of the file GetImage.c 
> <https://gitlab.freedesktop.org/xorg/lib/libx11/-/blob/master/src/GetImage.c> and got 
> confused about the function XGetImage. Specifically, I'm confused about where 
> the defined variable registerxGetImageReq*req; is used. The function never uses 
> it again after setting values for req at line 83 
> <https://gitlab.freedesktop.org/xorg/lib/libx11/-/blob/master/src/GetImage.c#L83>. 

The GetReq() call at line 73 sets up req to be in the Xlib buffer of requests to
send to the X server.  The _XReply call at line 85 causes everything in that
buffer, including req, to be sent to the X server for processing, so it
implicitly processes the contents of req.

> At line 85 
> <https://gitlab.freedesktop.org/xorg/lib/libx11/-/blob/master/src/GetImage.c#L85>, 
> does the function _XReply copy the whole display/screen's data to rep or just 
> the specified rectangle defined by req? If _XReply copies the whole 
> display/screen's data, where does the function XGetImage crop the image to keep 
> the specified rectangle only?

The function _XReply simply sends the message to the X server, who does all the
processing and returns the reply.  The X server is responsible for copying just
the specified area, not the whole screen, so Xlib does not need to do any
cropping on its own.

-- 
    -Alan Coopersmith-              alan.coopersmith at oracle.com
    Oracle Solaris Engineering - https://blogs.oracle.com/alanc
  
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