[GRASS5] PNG driver problem

David Piasecki piasecda at nv.doe.gov
Mon Aug 23 10:59:44 EDT 2004

Excellent! I'll give that a try! Leaving the PNG driver open would 
definitely solve this problem. I don't have a need to close it anyway. 
I just have to write the result to a named pipe.


On Aug 23, 2004, at 7:44 PM, Glynn Clements wrote:

> David Piasecki wrote:
>> I am using the PNG graphics driver in a C++ application that I am
>> writing. At the click of a button, I can zoom in and out by starting
>> the PNG driver, sending region and display commands to GRASS, and
>> stopping the PNG driver. When I behave like a normal user, it seems to
>> work just fine. However, when I try to break my code by clicking the
>> zoom button about 30-50 times in quick succession, I will eventually
>> get the message that the PNG driver is already running, even though 
>> the
>> output to the terminal window has verified that the PNG driver has
>> terminated. Is it possible to get such an error message if "d.mon
>> start=PNG" is called too quickly after "d.mon stop=PNG"?
> d.mon stop=..." runs mon.stop, which calls R_kill_driver() then exits.
> R_kill_driver() sends a GRAPH_CLOSE code to the driver, then calls
> R_release_driver(), which close()s the socket then returns.
> Nothing in this chain waits for the driver to close its end of the
> connection, so the driver could still be running after "d.mon
> stop=..." has completed (this is more likely to occur with the PNG
> driver, as it has to write out the image before it terminates).
> BTW, the latest CVS version of the PNG driver has an option to write
> the image file after each command. This eliminates the need to stop
> and re-start the driver in order to get the image. It also allows you
> to create (binary) PPM images as an alternative to PNG.
> -- 
> Glynn Clements <glynn.clements at virgin.net>

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