C++ header implementation

Question

Hello,
I've had this question before, but now I'm trying it from a new
approach. THe first time I tried to implement this headerfile in
LABVIEW 6.1 I tried a direct approach, but that didn't work out well.
My project is to implement the GetParmType stucture from this headerfile into a LABVIEW application.
The approach I'm trying at this moment is to build an array of 326 bytes, and fill this array with the data from my EEPROM.
The first 64Bytes is :        
char           
Version[VERSION_LEN];
the next 2 bytes are the:    unsigned short  DeviceId;
(this is the DeviceInfoType structure)
the next byte is:                unsigned char   NrChannels;
then 2 bytes for:               unsigned short  NrPixels;
and 1 byte for:           
      unsigned char   Sensor;
(this is the DeviceConfigType
the rest of the array should be filled with the ChannelParmType
structure. The maximum devices that can be connected is 8 pieces, so
this should be a loop that runs the 8 times.
Can somebody help me with this? I have no idea how this is done or how to fill an array this way.
The headerfile is attached.
Many thanx
Koert
Attachments:
avs_dll.h ‏61 KB

Answer 1

Lycangeek wrote:
The DeviceConfigType is a great example of what I mentioned before about the dangers of mapping C structures to LV...   typedef struct
    unsigned char   NrChannels;         // number of channels
unsigned short 
NrPixels;          
// number of pixels
    unsigned char  
Sensor;            
// Sensortype
} DeviceConfigType;
Typically in C/C++, you are going to have the compiler set to a 4-byte alignment (this is the default for 32-bit Windows).
I
think Visual C (at least in version 6) uses a default alignement of 8
byte. However variables are usually aligned to the lesser of the
default alignement or the variable size. So I would hazard that this
particular structure actually would take up 5 byte since Visual C will
add a fill byte between NrChannels and NrPixels.
However you are right that in my example I didn't account for padding
at all. Another problem is that there will be three additional fill
bytes after Sensor, to make the first float in the ChannelArray align
on a four byte boundary.
So this would probably mean that for the DeviceConfig we should copy
out at least 7 bytes and add a fill byte between channels and pixels
and the offset for the channel array should be 74 instead of 70. Also
the array intialization should then be 330 instead of 326.
There still are some uncertainities. Unless the #pragma pack(x)
statement is in the header file (in this case not) you still can't be
certain that the DLL was compiled with the default alignment, although
it is quite likely.
Rolf Kalbermatter
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions