How to pass a struct to a DLL function and accessing it in another VI

Question

Hi friends,
                   I am new to labview. I need to create a demo program in labview ,for displaying image from our own image capturing system. We have a  DLL ( build in VC++) containing functions for capturing image from our system. Now I need to create a VI library for some of functions in DLL and Create a Demo program using those created subvi library . I used "Call Function node" and created some of subvi's.
 Some of our DLL functions need to pass struct pointers.  Our function prototype will be similar to the following function.
__declspec(dllexport) int __stdcall Initialize( unsigned char *imagebuffer,struct config *Configuration);
The passed struct is similar to
struct config
  double                val1[3];
  unsigned short   val2;
  bool                    val3;
  bool                    val4[3];    
  unsigned char    val5;    
  unsigned char   val6[3];
  bool                    val7[26];
For passing "unsigned char *imagebuffer"  I initialized array with "Numeric constant " and set the size of the array and send to the function.
The problem here is, I used this array in one of the subvi. 
How can I use the returned imagebuffer array  in my main demo program. How to connect the image array to subvi "Connecter Pane"
And  which control  can I use to display the image. The image data I get is form of 1-D Array .
The second problem is,
                             For passing the structure,  I used "Bundle " and filled the bundle with all the datatypes as in my struct and passed to the function. Is it correct ?  How to access this bundle after returned from function  in another Vi. ie.) How to connect this bundle to the connter pane ?
Thanks for your valuable suggestions.
aajjf.
Message Edited by aajjf on 04-19-2007 05:34 AM

Answer 1

aajjf wrote:
Hi friends,
                   I am new to labview. I need to create a demo program in labview ,for displaying image from our own image capturing system. We have a  DLL ( build in VC++) containing functions for capturing image from our system. Now I need to create a VI library for some of functions in DLL and Create a Demo program using those created subvi library . I used "Call Function node" and created some of subvi's.
 Some of our DLL functions need to pass struct pointers.  Our function prototype will be similar to the following function.
__declspec(dllexport) int __stdcall Initialize( unsigned char *imagebuffer,struct config *Configuration);
The passed struct is similar to
struct config
  double                val1[3];
  unsigned short   val2;
  bool                    val3;
  bool                    val4[3];    
  unsigned char    val5;    
  unsigned char   val6[3];
  bool                    val7[26];
For passing "unsigned char *imagebuffer"  I initialized array with "Numeric constant " and set the size of the array and send to the function.
The problem here is, I used this array in one of the subvi. 
How can I use the returned imagebuffer array  in my main demo program. How to connect the image array to subvi "Connecter Pane"
And  which control  can I use to display the image. The image data I get is form of 1-D Array .
The second problem is,
                             For passing the structure,  I used "Bundle " and filled the bundle with all the datatypes as in my struct and passed to the function. Is it correct ?  How to access this bundle after returned from function  in another Vi. ie.) How to connect this bundle to the connter pane ?
Thanks for your valuable suggestions.
aajjf.
Message Edited by aajjf on 04-19-2007 05:34 AM
You say nothing about how your cluster looks but I'm afraid you did the standard error here and placed arrays in it. That is not what the C structure is representing for several reasons.
First fixed size arrays in C are inlined inside a structure, so are not a pointer but for the case of your val1 element three doubles.
Second although not relevant here because of above point: LabVIEW arrays are not the same as C arrays. LabVIEW uses a pointer to a pointer and has the size of the array in elements prepended to the array data. C simply uses a pointer and all the rest is the programmers sorrow. The Call Library Node does convert the top level element of variables you pass according to the type configuration of that parameter but does no conversion of internal elements at all. So passing clusters with anything like arrays or strings is always wrong unless the DLL is aware of the specific datatypes LabVIEW uses.
Last but not least you can end up with alignment issues. Elements in structures are aligned by every C compiler to a certain value. This value can be defined by the programmer in the project settings or for Visual C through #pragma pack() statements in the C code. The alignment rule says that an variable is aligned to the smaller of the two values that are either a multiple of the variable element size or the alignment setting. Most 32bit code nowadays uses 8 bit default alignment but LabVIEW always uses 1 byte. In your case there is nothing to observe since the large variables are at the beginning. Otherwise you might have had to insert filler elements in the LabVIEW cluster.
One last thing bool is a C++ type only. Its size is 1 byte and incidentially this is the same LabVIEW uses for Booleans.
Rolf Kalbermatter
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions