[cups-devel] The difference of data sent on macOS and Linux

[DanTan at pek.destiny.com.cn]

Dear Sam,

Thank you for your reply. I have tried your suggest, but it doesn’t work.

On Aug 10, 2016, at 1:01 AM, Sam Hanes <sam at maltera.com<mailto:sam at maltera.com>> wrote:

On 08/09/2016 03:42 AM, DanTan at pek.destiny.com.cn<mailto:DanTan at pek.destiny.com.cn> wrote:
I am developing a raster driver (filter) with same code on both macOS
and Linux. In the code I flush stdout every time with a page end. I
wish CUPS will send the page data to printer, so when the driver
handing the next page, the printer can print the current page. The
design is good on Linux, but on macOS, CUPS didn’t send the data,
until filter exit (Check on macOS 10.10.5).

Flushing stdout in your filter will ensure that all the data you've
written up to that point is sent along to the next filter in the
pipeline, but it doesn't force any subsequent filters to flush *their*
buffers so it won't necessarily result in getting everything sent to the
printer.

So, I wonder, if there is a way for filter to force CUPS to send the
data to printer on macOS, or it just a different behavior on
different OS.

If you're the last filter before the backend (which I think should be
the case if you're writing a raster driver and don't use a port
monitor), you can cause the backend to flush its buffers with the
`DRAIN_OUTPUT` command via `cupsSideChannelDoRequest`.

For more info on that, see the documentation:
https://www.cups.org/documentation.php/doc-2.1/api-filter.html#DRAIN_OUTPUT
https://www.cups.org/documentation.php/doc-2.1/api-filter.html#cupsSideChannelDoRequest

Yes, I have tried cupsSideChannelDoRequest with DRAIN_OUTPUT, but it doesn’t work yet.
CUPS debug log as follow, it shows the command is OK.
……….
D [10/Aug/2016:13:41:51 +0800] [Job 77] Read 660 bytes of print data...
D [10/Aug/2016:13:41:51 +0800] [Job 77] Wrote 660 bytes of print data...
D [10/Aug/2016:13:41:51 +0800] [Job 77] backendDrainOutput(print_fd=0, device_fd=8)
……...



If you're not the last filter before the backend I'm not aware of any
way to force a flush of the entire filter pipeline. I'm just a filter
developer, though, not a member of the CUPS project, so one of them may
get back to you with more information.

As the "Figure 1: Raster Filter Chain" in https://www.cups.org/documentation.php/doc-2.1/raster-driver.html#BASICS,
except the “Optional Port Monitor”, the raster filter should be the last filter before backend.
--
Sam Hanes
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===========================
Unus pro omnibus, omnes pro uno
-Dan✉


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