LabVIEW
C structures/pointers with labview clusters
Hi,
You should clearly understand, how data stored in the memory.
Question for you:
You have structure like this:
Can you say which value will return sizeof()? You may think - 46 bytes (6x1 + 2*4 + 8*4). Wrong! Right answer is 48, because between char b6 and long l1 you have two bytes gap for alignment.
Well, if DLL is your DLL, then you can add #pragma pack(1), then you will have no gap. But if your DLL is not yours, then you should add according gap in LabVIEW cluster:
Also take a note, that arrays stored in structures indirectly, as result cluster with array will not work for you. But LabVIEW stores clusters embedded within other clusters directly in line and with no indirection, so you can replace your array with cluster with appropriate number of the elements as shown in post above.
Hope it helps,
Andrey.
Andrey Dmitriev,
Here I have a doubt. What if the structure you shown has enum instead of char? As I learned in other forums that, compiler allocates memory for an enum datatype (it could be 1byte or 2bytes or 4bytes) based on someother thing, how can we predict the number of gaps we need to add for a cluster (which equivalent to that C structure)?
-Ajay.
with enum - in general same as with numeric.
What you can do for understanding how LabVIEW stores data in memory is pretty simple and straight forward test - use MoveBlock function for transferring data from cluster into byte array:
now you can see that U16 enum takes two bytes as expected.
Be careful - if your cluster is "shorter" than the destination, the you will read memory outside of cluster - therefore exception may occur.
Andrey.
Andrey Dmitriev,
That's cool. But now I want to reference a structure as follows,
struct XXX
{
int AAA;
char BBB[10];
float CCC[40];
float DDD[40];
int32 EEE;
int32 FFF;
}
DLL Header:
ER_CODE KMAPI CLSetRankData(DEVICE_HANDLE hDevice, int32_km DataNo, const CL_RANK_DATA* RankData);
where the typedef of CL_RANK_DATA is as follows,
typedef struct tCL_RANK_DATA
{
CL_RANK_TYPE Type;
int8_km RankName[CL_RANK_NAMESIZE_SAVE];
CL_xyDATA xyPoint[CL_RANK_POINTNUM];
CL_TcpduvDATA TcpduvPoint[CL_RANK_POINTNUM];
int32_km PointNum;
int32_km Enable;
}
CL_RANK_DATA;
You can find that I have int8_km in the typedef is a string. How can I construct a LabVIEW cluster which defines this structure? Also notice that I need to pass the pointer of the cluster to the method.
Kindly let me know how to handle this in LabVIEW.
Thanks,
Ajay
