| Re: Passing Structures with Pointers with Call_External [message #40459 is a reply to message #40458] |
Wed, 11 August 2004 06:53   |
MajorSetback
Messages: 9
Registered: August 2004
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Junior Member |
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"Peter Mason" <drone@spam.com> wrote in message news:<vDcSc.266$aA.11145@news.optus.net.au>...
> PeterOut wrote:
> <...>
>>
> temp={Rows:long(numrows),Columns:long(numcolumns),Data:fltar r(numrows,numcol
> umns)}
> <...>
>> The C code is as follows.
>> typedef struct FloatPlane_Struct
>> {
>> long Rows;
>> long Columns;
>> float **Data;
>> } FloatPlane;
> <...>
>> If I add
>> fprintf(stderr, "fppPlanes->Data[0]=%d\n", fppPlanes->Data[0]);
>> idlde crashes, presumably due to a memory write error in the C code.
>> Is there any way to stop idlde crashing under such circumstances?
>>
>> My main question is this. Is there a way to retrieve the IDL variable
>> Planes[i].Data within CFunction_cw?
>
>
> The problem here is that IDL isn't creating the structure quite as you
> expect. There isn't that level of indirection with DATA. Your C-side
> structure should look something like this:
> typedef struct FloatPlane_Struct {
> int Rows;
> int Columns;
> float Data[n];
> } FloatPlane;
> Where the "n" in "Data[n]" is equal to numrows*numcolumns in your IDL-side
> structure creation statement.
> I think this means that you need a different approach as a C-side structure
> definition is fixed at compile time ("n" must be a constant).
>
> You might be wondering about changing your structure definition to use an
> IDL "pointer" for the array? Don't even try it. The value of an IDL
> pointer is like some handle index thing and bears no relation to an actual
> memory address. It's meaningless to external code.
>
> Personally, I'd suggest abandoning the use of a structure and coding a DLM
> instead of CALL_EXTERNAL here. CALL_EXTERNAL is quick and easy but
> sometimes it's worth going that extra distance. In a DLM you would be able
> to pull out the dimensions of your DATA array (now 3-dimensional for the
> frames). Also, the IDL-side work would probably be more efficient with a
> straightforward array instead of arrays embedded in structures.
> Alternatively, stick to CALL_EXTERNAL and pass your C function two
> parameters: DATA and SIZE(DATA).
>
> Peter Mason
Hi Peter,
Thanks very much for the reply. I decided to do something along the
lines of the last thing you mentioned. I made a 3D volume with IDL
thus.
Volume=fltarr(nPlanes,nRows,nCols)
I then filled the volume with the plane data and pass it thus.
Result=Call_External('SharedLibrary.so','CFunction_cw',NumPl anes,NumRows,NumCols,Volume,/unload)
In the C function, I then have a float pointer that reads the array
thus.
float *fpData=((float *)(argv[3]));
Hence I have a pointer to the data, in 1D, that can be upwrapped using
the volumetric dimensions.
Thanks again for your help,
Peter.
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