| C Alignment/IDL structures [message #43098] |
Wed, 16 March 2005 08:13  |
joey
Messages: 5
Registered: March 2005
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Junior Member |
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Hi! I have an C library which I link into IDL using IDL_MakeStruct. The
structure I link in with IDL is not a true C structure, but a dynamic
structure I create on the fly depending on the data I wish to have within
IDL.
It works quite well; however, one of the elements incorporated in my
structure is another structure which I would like to add a double value.
When I add this double, my structure size seems to get the wrong size for
IDL to handle so nothing will work when accessing the structure. I compute
the size to a value which is the sum of all bytes in the structure which
obviously does not take into account the packing/aligning that C does.
My question is: is there a way I can figure out how many bytes to malloc
such that IDL and I will always be in agreement?
Even if we are in agreement, I need to know where the padding will occur so
I would like to have the same algorithm that IDL uses to compute its
structure allocation when given a list of tags.
If I stick to floats/long/int/etc/strings/etc. everything seems to work
great. Its only when adding the double that everything goes bad.
Thanks for any advice!
Joey
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| Re: C Alignment/IDL structures [message #43159 is a reply to message #43098] |
Mon, 21 March 2005 08:56  |
joey
Messages: 5
Registered: March 2005
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Junior Member |
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Randall Skelton <randall.skelton@gmail.com> wrote:
> I'm not sure I completely understand what you are doing... can you post
> a snipit of code? I trust that you are passing back an unnamed
Here's my code that does the IDL/C interaction. I have a vector (_dataIDL)
which has all the values I want into IDL within it as a malloc'ed sets of
memory.
// Copy the real data
unsigned long pos = 0;
unsigned char *myStructureThatLooksLikeTags = malloc (_totalSpaceNeeded
* _dataIDL.size ());
for (unsigned int i = 0; i < _dataIDL.size (); i++) {
memcpy (&(myStructureThatLooksLikeTags [pos]), _dataIDL [i],
_totalSpaceNeeded);
pos += _totalSpaceNeeded;
}
char idl_struct_name [100];
sprintf (idl_struct_name, "%s_K%ld_V%d", dbVirtualName (data_key),
data_key, version);
void *idl_struct = IDL_MakeStruct (idl_struct_name, tags);
IDL_LONG dims;
dims = _dataIDL.size (); // number of elment in the array of structures
IDL_VPTR ivReturn = IDL_ImportArray (1, &dims, IDL_TYP_STRUCT,
myStructureThatLooksLikeTags,
cleanUpIDL, idl_struct);
> In general, when I have to pass C structures from existing code back to
> IDL I do it by creating a shadow structure (in C) that uses all the
> defined IDL types and copying the data. You really cannot rely on
> generic C variables having the same size as thier IDL counterparts
> (take a look at the definition of IDL_ALLTYPES in idl_export.h).
Ok, this probably answers my question. I was hoping I could create an array
of structs, but this is maybe not so memory efficient so I might try to
create one structure with multiple arrays.
Joey
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| Re: C Alignment/IDL structures [message #43195 is a reply to message #43098] |
Wed, 16 March 2005 10:07 |
Randall Skelton
Messages: 31
Registered: October 2004
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Member |
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I'm not sure I completely understand what you are doing... can you post
a snipit of code? I trust that you are passing back an unnamed
structure from C as named structures are persistent in a given session.
Hence, you should call as:
ptr = IDL_MakeStruct(NULL, mytags);
This is the reason the following fails:
IDL> a = {foo, a: 0, b: ''}
IDL> b = {foo, c: 0}
% Wrong number of tags defined for structure: FOO.
Once 'foo' is defined, you cannot change its fields.
In general, when I have to pass C structures from existing code back to
IDL I do it by creating a shadow structure (in C) that uses all the
defined IDL types and copying the data. You really cannot rely on
generic C variables having the same size as thier IDL counterparts
(take a look at the definition of IDL_ALLTYPES in idl_export.h). You
can cheat a little when initializing arrays as you can point to the
memory in your existing structures if the sizes align, but everything
in the C structure should be defined with one of the macros given in
the EDG. I don't think you should be seeing any structure alignment
issues...
Cheers,
Randall
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