| Re: Returning a struct containing variable-length arrays [message #62527 is a reply to message #62521] |
Wed, 17 September 2008 01:46   |
Nigel Wade
Messages: 286
Registered: March 1998
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Senior Member |
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franzpx125 wrote:
> Hi!
>
> Within my DLM, I need to return a struct containing two arrays but the
> length of the arrays is known only at run-time. Which is the correct
> way to write the DLM code? I think I have to use IDL_MAX_ARRAY_DIM and
> I defined:
>
> static IDL_MEMINT my_tags_dims[] = { 1, IDL_MAX_ARRAY_DIM };
>
> static IDL_STRUCT_TAG_DEF my_tags[] = {
> { "X", my_tags_dims, (void *) IDL_TYP_DOUBLE },
> { "Y", my_tags_dims, (void *) IDL_TYP_DOUBLE },
> { 0 }
> };
>
> but I don't know how to fill the fields of this struct. Any help?
>
> Thanks,
> Brun Francesco
You basically have two choices depending on whether you want to use your own
memory storage allocated by your DLM code to hold the structure data, or have
IDL allocate its own memory and copy the values into that. Each method has its
own set of advantages and caveats/gotchas.
If you use your own memory allocation then that memory has to persist after the
DLM routine returns. This either means it has to be declared static (but since
you want to use variable length arrays this isn't possible) or allocated by
malloc. Using this method means that you are responsible for freeing the memory
when IDL has finished with it. If it is simple throw away code which will only
be used occasionally, and the function isn't called many times, and doesn't
allocate much memory, you can ignore this but you *will* have a memory leak. To
cure this you need to implement the callback routine, a function pointer to
this is passed into the IDL_ImportArray function so that it can free whatever
memory you allocate.
If you create an IDL struct using your own memory then you don't have to worry
about copying the data into the structure. If you use IDL_MakeStruct so that
IDL allocates the storage (and handles de-allocation) then you do have to copy
the data into that memory. This is a little tricky.
In the first case, allocating your own storage, the sequence is to first
determine how much storage is required. Then you set your dimensions arrays
appropriately. Next you create the structure with IDL_MakeStruct and finally
import your data into the structure using IDL_ImportArray. You are responsible
for writing a callback function to free this memory when IDL has finished with
the variable.
To get IDL to handle the allocation the first two steps are the same. Then,
rather than importing your own data you use IDL_MakeTempStruct to create a IDL
temporary variable (which you can pass back to IDL). The tricky part is
determining where to put your data. To do this you can use the function
IDL_StructTagInfoByName, which returns the byte offset into the struct where
the data for that tag is located. Then it's a case of some hairy pointer
manipulation to write the data into the structure.
--
Nigel Wade
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