A SANE scanner backend written in Python?

dwelch donald.welch at hp.com
Wed Mar 29 01:20:21 CEST 2006


I've read many times on this newsgroup over the years that you can write 
"just about anything" in Python, except perhaps a full operating system 
(maybe even that...?). So, with this spirit in mind, I would like to try 
to write a SANE scanner backend that I need to write, in Python rather 
than C. This work is being done on Linux using Python 2.4.2.

If you are not familar with SANE, and the SANE API, it basically boils 
down to supplying a libsane-<name of your backend>.so shared library 
that supports a basic C interface. Example sane API calls into a backend 
are: sane_init(), sane_open(), sane_start(), sane_read() and 
sane_cancel(). At runtime, a backend is loaded up and its C interface is 
called to determine what devices are supported by the backend, to set 
options, and to actually perform a scan.

Obviously, this is an example of embedding Python, albiet with a 
different end result. I have not found any examples in the wild that 
embed Python in a shared library (vs. a standalone executable).

My trials so far have been fruitless. To start with, I simply took an 
outline of a shared library that defined all the relevant SANE APIs, 
linked it with SANE and Python, and placed a PyInitialize() in the 
sane_init() function. This immediately results in a segmentation fault. 
I'm not sure how to proceed from here. How would a separate .py file be 
accessed in this way and how would I call "back" into Python code when a 
C API was called? Is there going to be a problem of maintaining "state" 
in Python between invocations of the C API?

I've also explored using Elmer for this task, but have not had luck so 
far. Its also not clear to me whether this tool is appropriate for this 
task.

So, what I'd be interested in knowing is whether this is a foolish 
venture, or if I just have the wrong approach.

Any thoughts would be appreciated.

Thanks,

-Don





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