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type: channel
description: Messages from Usenet group comp.lang.idl-pvwave, compiled by Paulo Penteado
items: C Alignment/IDL structures
items: Re: C Alignment/IDL structures

C Alignment/IDL structures
type: item
description: 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
creator: joey
date: 2005-03-16T16:13:03-00:00

Re: C Alignment/IDL structures
type: item
description: 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
creator: Randall Skelton
date: 2005-03-16T18:07:33-00:00

Re: C Alignment/IDL structures
type: item
description: 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
creator: joey
date: 2005-03-21T16:56:20-00:00