[Gdal-dev] Re: SWIG and CSharp

[Kevin Ruland]

Frank,

Welcome to the club!

Frank Warmerdam wrote:

>Kevin,
>
>I have plunged into the world of SWIG, and with my choice of
>CSharp I seem to have picked the deep end. :-)
>
>  
>
Yes, it's very deep.  There is a new SWIG 1.3.26rc1 out right now.  It
was cut from cvs a couple of days ago (Saturday, I think).  In any case
there have been changes to C# in 1.3.26 which might prove useful.  In
particular, they've fixed the naming of the setter/getter functions for
the member variables so we can remove the renamed setters/getters from
the gdal_cshap.i file.  What I mean, they use the same naming as the
python and the other generators.

My understanding of C# is it should be relatively easy to make these
typemaps because you don't have to do a bunch of magic to converts lists
to/from arrays etc.

> o I have implemented typemaps for "double *" and "double arg[ANY]"
>    that seem good enough that stuff like GetGeoTransform(),
>    and SetGeoTransform() are now working.  I just map it to a double[]
>    in c#.  Currently I don't think this distinguishes the case where a
>    pointer to a single double is used to return values so there may be
>    more work to do.   Also, there is no array size checking, so if
>    the app passes too small a c# array memory will get corrupted.
>
>  
>
There is a distinction made in the typemaps for return arguments and for
arrays. These are used for output arguments:

%typemap(in,numinputs=0) (double argout[ANY])
%typemap(argout) (double argout[ANY])

%typemaps(in,numinputs=0) (double *argout[ANY])
%typemaps(argout) (double *argout[ANY])
%typemap(freearg) (double *argout[ANY])

The grouping is intentional.  The 'in,numinputs=0' piece is used at the
top of the wrapper, the 'argout' is used to convert the returned value
from the wrapped method into whatever is really returned by the wrapper,
the 'freearg' is used to release memory allocated by the
'in,numinputs=0' typemap.

This one is used for input:

%typemap(in) (double argin[ANY])

These typemaps will match functions which have this kind of signature:

void funcout( double argout[14] )
void funcpout( double *argout[7] )
void funin( double argin[3] )

etc.  In particular, it requires the argument to be a fixed sized array.

There is only one typemap used for arbitrary double * parameters:

%typemap(xxx) (double *argout)

In the python bindings, we used the 'double *OUTPUT' typemap which comes
from the typemaps.i library.

There may or may not be a way to catch the small array problem.  In the
wrappers for the [ANY] types, you have access to the size required in
$dim0 (or $1_dim0 if you have more than one argument).  However, I don't
know if you have access to the size sent in from C#.

> o I am working on passing buffers for ReadRaster() and WriteRaster().
>    I have the WriteRaster case on the dataset working (I think) with
>    the following magic:
>
>%typemap(ctype)  (int buf_len, char *buf_string) "int, char *"
>%typemap(imtype) (int buf_len, char *buf_string) "int buf_len, byte[]"
>%typemap(cstype) (int buf_len, char *buf_string) "byte[]"
>%typemap(csin)   (int buf_len, char *buf_string) "$csinput.Length, $csinput"
>
>    I have used the specific names of the arguments in the hope to
>    restrict it to the case of buffers with a length being passed in to
>    stuff like WriteRaster().  This now applies to Dataset.WriteRaster(),
>    but not too Band.WriteRaster().  Would it be ok for me to change
>    Band.i to use the exact same argument names for buf_len and buf_string
>    as Dataset.i?   I forsee some other specific naming conventions being
>    needed for some other cases that I want to handle pretty specially  in
>    c#.  Other than obvious special names like argin and argout how much
>    dependence is there on argument names in the .i files?
>
>  
>
I think in your follow-up email you touched on this.  We used named
typemaps and use the %apply/%clear to make sure the proper typemap is
applied to each function.  I think was a reasonable solution which
provides complete control over which typemaps get applied although it is
a little more cumbersome.  You need to follow the typemap argument names
outlined in the README.typemaps file.  Those argument names are used in
the %apply commands.

I don't really know what the special c# typemaps are.  Here you've used
'ctype', 'imtype', 'cstype', 'csin'.  They are particular to C# wrapper
generation.  I hope they get used along with the 'in', 'out', 'argout'
typemaps when we use %apply.

>There are still lots of mappings left to do, but I would like to at least
>get the point where I can do various GDAL operations, and then review
>OGR after that.
>
>I have been able to open datasets, get back stuff like their size,
>projection and now geotransform.  I haven't gotten much further than
>that.
>
>  
>
That's further than before :)  One small step....

Kevin