| Re: C Alignment/IDL structures [message #43068] |
Wed, 23 March 2005 08:20  |
joey
Messages: 5
Registered: March 2005
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Junior Member |
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Nigel Wade <nmw@ion.le.ac.uk> wrote:
>>
>> // 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;
>> }
>>
> In your code you copy memory from two disimilar data structures using
> memcpy() taking no account of alignment. Your memory allocation uses the
> construct _dataIDL.size(), what is that? Since you haven't shown us the
_dataIDL is actually a C++ vector of unsigned char *'s. This group of
char * is created from a C++ map of data. The map is a tagname and data
value. Complex, isn't it? I'd like to think its elegant since I can very
simply wrap any C/C++ and interface it with IDL. However, if one of the
items in the map is a structure itself, I think it has problem.
Randall: I got your code and I am going to look at it. I appreciate the
example! I am going to try throwing a structure with a double in there
and see if I can generate the same problem I am seeing on my code.
Cheers,
Joey
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| Re: C Alignment/IDL structures [message #43075 is a reply to message #43068] |
Tue, 22 March 2005 02:34  |
Nigel Wade
Messages: 286
Registered: March 1998
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Senior Member |
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joey@swri.edu wrote:
> 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
It is possible to do what you want to do. But you have to realize that there
are some caveats, and they are pretty serious ones.
The padding within a C structure is not defined by the language. It is an
implementation issue, and is entirely at the discretion of the compiler
developer. There is no guarantee that the padding inserted by one C
compiler will be the same as the padding inserted by another compiler. You
can't even guarantee that the next minor release of your compiler will use
the same padding as the current release. It's a pretty safe bet, but not
guaranteed.
In your code you copy memory from two disimilar data structures using
memcpy() taking no account of alignment. Your memory allocation uses the
construct _dataIDL.size(), what is that? Since you haven't shown us the
definition of the storage for _dataIDL or your IDL tags, then it's
impossible to say what's wrong. But my guess would be your data types or
alignment in _dataIDL doesn't match what you've specified in your IDL
structure tags.
It is possible to generate arrays of structures in C and pass them back to
IDL, but you have to be very careful and you have to undertand its
limitations.
--
Nigel Wade, System Administrator, Space Plasma Physics Group,
University of Leicester, Leicester, LE1 7RH, UK
E-mail : nmw@ion.le.ac.uk
Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555
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| Re: C Alignment/IDL structures [message #43334 is a reply to message #43068] |
Thu, 31 March 2005 02:46 |
Nigel Wade
Messages: 286
Registered: March 1998
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Senior Member |
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joey@swri.edu wrote:
> Nigel Wade <nmw@ion.le.ac.uk> wrote:
>>>
>>> // 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;
>>> }
>>>
>
>> In your code you copy memory from two disimilar data structures using
>> memcpy() taking no account of alignment. Your memory allocation uses the
>> construct _dataIDL.size(), what is that? Since you haven't shown us the
>
> _dataIDL is actually a C++ vector of unsigned char *'s. This group of
> char * is created from a C++ map of data. The map is a tagname and data
> value. Complex, isn't it? I'd like to think its elegant since I can very
> simply wrap any C/C++ and interface it with IDL. However, if one of the
> items in the map is a structure itself, I think it has problem.
Perhaps it's overly complicated?
You have an array of tags called tags which you pass into IDL_MakeStruct.
These are the tag names which will be set in the structure it creates. Why
does the data structure _dataIDL also have tag names in it? I'm confused.
Are you trying to create a structure which contains a structure?. If so,
what I do is set the tag type in the IDL_STRUCT_TAG_DEF array for the
parent structure to IDL_TYP_STRUCT. For the child structure I use
IDL_MakeStruct to create it's IDL structure, and then set the parent
structure type for that tag to be this value.
E.g. this creates a structure containing a structure. The display_tags
structure contains a structure with protocol_tags.
IDL_STRUCT_TAG_DEF protocol_tags[] = {
{ "VERSION", 0, (void *)IDL_TYP_LONG },
{ "REVISION", 0, (void *)IDL_TYP_LONG },
{ 0 },
};
IDL_STRUCT_TAG_DEF display_tags[] = {
{ "NAME", 0, (void *)IDL_TYP_STRING },
{ "VENDOR", 0, (void *)IDL_TYP_STRING },
{ "PROTOCOL", 0, (void *)IDL_TYP_STRUCT },
/* set this to protocol_s later */
{ 0 },
};
void *protocol_s, *display_s;
protocol_s = IDL_MakeStruct(0, protocol_tags);
display_tags[2].type = protocol_s;
display_s = IDL_MakeStruct(0, display_tags);
IDL_TYP_STRUCT in the tags array is really just a place-holder and comment,
it doesn't do anything as it's replaced before the tags array is used to
create the parent structure.
--
Nigel Wade, System Administrator, Space Plasma Physics Group,
University of Leicester, Leicester, LE1 7RH, UK
E-mail : nmw@ion.le.ac.uk
Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555
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