Is it possible to access a pointer to a structure within another structure via libpointer?

2012 9 21
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Hello,
I am using an external c/c++ shared library from within MATLAB.
Please see the code below:
% c/c++ interface function
% bool oa_get_pattern_objects ( int instance_id , oa_object_array * pReturned_objects )
function [found object_xyrecords] = get_pattern_objects(instance_id)
oa_object_array.m_objects = {};
oa_object_array.m_size = 0;
pOA_pattern_objects = libpointer('oa_object_array',oa_object_array);
if calllib('OpenAccessCore','oa_get_pattern_objects',instance_id,pOA_pattern_objects) == true
found = true;
proxy_objects = get(pOA_pattern_objects,'Value');
size = proxy_objects.m_size;
%THIS IS ILLEGAL
setdatatype(proxy_objects.m_objects,'oa_xyrecordPtr',1,size)
object_xyrecords = proxy_objects.m_objects.Value;
else
found = false;
object_xyrecords = [];
end
I cannot use setdatatype to inform MATLAB that the field, m_objects, points to an array of structures of the same type.
Perhaps I could get a pointer to the first element in the array and manually convert them in MATLAB one-by-one. Is there a way to do this?
My real desire is to give MATLAB the information it needs and allow it to convert all the elements for me more efficiently and elegantly than I can. Is there a way I can do this?
For a simple c-structure like below is it possible to access the elements in the array pointed to by the pointer, sc_array, by any means in MATLAB?
struct simple_class
{
int my_id;
simple_class * sc_array;
};
Any thoughts are greatly appreciated :-)

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shon
shon 2012-9-25
编辑：shon 2012-10-2

0 个投票

I've discovered a solution that works but I don't really understand why that is.
I can create records in which a record may contain different structures. If these records are to be initialized in C/C++ then structures must precede any PODs in the record's definition.
struct file
{
char *path_;
}
struct record
{
file f_;
int file_size_; // POD cannot be defined before any struct
}
MATLAB appears to be "reshaping" the memory for structs in MATLAB code.
If a struct is placed after a POD then it's fields are not aligned in memory as I would desire as I cannot access the fields of that struct and confirm the right data is where it should be.
If I define all the structs before PODs then for some reason everything works out okay.
This answer is incomplete as I do not fully understand what is happening or why but instead have only discovered how to circumvent the affects of what is happening so that I can work with MATLAB. It would be wonderful to understand just what IS going on.

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Philip Borghesani
Philip Borghesani 2012-9-27
编辑：Philip Borghesani 2012-9-27
What release of MATLAB are you using? There have been alignment bugs with structure layout when there were imbedded structure. I believe they were all fixed by release R2010a.
It may be possible for you to work around this by specifing a structure packing in your header file start with 8 but you can try other values:
#pragma pack 8
Philip Borghesani
Philip Borghesani 2012-9-27
Walter POD is a c/c++term it stands for "plain old data" but has a much more specific meaning in the specifications.
shon
shon 2012-10-2
Thank you Philip. I'm using the "plain old data" definition for POD.
I've been using R2012a.
It may be possible to use #pragma pack 8 to set my alignment boundaries. I had not thought of that. Thank you. I'll try to keep this in mind going forward but I'm still a little weary.
What is MATLAB doing to my memory or should I be asking how is MATLAB interpreting my memory layout?

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