Conversion troubles

[orko]

Hello all,

I'm familiar with passing simple integer and string values into CINs, but I'm just getting started with passing structures and would appreciate a helping hand.

My dll has a structure as follows:

typedef struct test

{

int num;

unsigned char name[4];

int dnum[16];

unsigned char flet;

char aname[16][256];

}

Test;

I'm trying to build a typedef'd cluster control that I can use to pass data in and out of a function in my DLL that uses the above. I had fairly good luck in doing this with another structure that held only integer returns (no arrays), but I haven't been able to get this one nailed down.

I believe I have the first three controls set up correctly in the typedef cluster:

Element1: I32

Element2: cluster of ( 1D array of U8 with field length set to min length=4, padded with zeros ) (can I just use a string here?)

Element3: cluster of ( 1D array of I32 with field length set to min length=16, padded with zeros )

Element4: U8

If this is wrong, please correct me. I may be confusing the field length minimum setting with the maximum entries in the array(?)

The fifth element being a 2D array of char is throwing me for a loop however. Since this is an array of char with each element having a max length of 256 and a max array size of 16 elements, I tried having 16 more elements in my typedef like the following:

Element5: cluster of ( 1D array of U8 with field length set to min length=256, padded with zeros )

...

Element20: cluster of ( 1D array of U8 with field length set to min length=256, padded with zeros )

LabView has crashed on me every time I tried to use this however, so I must not have this set up correctly.

I've attached my typedef (LV 8.0.1). Could someone point me in the right direction (help stamp out the obvious bugs)?

Download File:post-3266-1150395023.ctl

-- orko

[bsvingen]

2D arrays in C are not "ordinary" 2D arrays, but an array of pointers to arrays (pointers to pointers to whatever). Labview stores 2D arrays as 1D contiguous block of memory, or as a 1D array. The only difference between 2D and 1D array in labview is an extra long int that hold the second dimension.

[Rolf Kalbermatter]

Hello all,

I'm familiar with passing simple integer and string values into CINs, but I'm just getting started with passing structures and would appreciate a helping hand.

My dll has a structure as follows:

typedef struct test

{

int num;

unsigned char name[4];

int dnum[16];

unsigned char flet;

char aname[16][256];

}

Test;

I'm trying to build a typedef'd cluster control that I can use to pass data in and out of a function in my DLL that uses the above. I had fairly good luck in doing this with another structure that held only integer returns (no arrays), but I haven't been able to get this one nailed down.

You got quite a bit wrong here!

First the arrays you have are all fixed size so they are inlined as such and represent nothing like a pointer.

You would need a cluster with following setup:

int32 num

cluster of 4 uInt8 name

cluster of 16 int32 dnum

uInt8 flet

cluster of 16 * 256 uInt8 aname

Especially the last one is not very convinient to do and I personally would treat the entire structure as an array of unsigned bytes passed as a C array pointer and write VIs that insert an retrieve the according information from that byte stream.

Rolf Kalbermatter

[orko]

You got quite a bit wrong here!

Yes, I realize it's all wrong. I have yet to wrap my brain around the differences between C and LabView data structures, so I decided to post what I had so far "kludged" together. Better in my mind than to post a generic, "can anyone give me my answer?" post.

First the arrays you have are all fixed size so they are inlined as such and represent nothing like a pointer.

Okay, arrays won't work for passing data inside the clusters because LabVeiw handles array data differently, correct? But I could use an array to build the data in LabView, then pass it to "Array to Cluster.vi" to build up the cluster you define below?

I am unfamiliar with what you mean by "inlined". Could you expound on that?

You would need a cluster with following setup:

int32 num

cluster of 4 uInt8 name

cluster of 16 int32 dnum

uInt8 flet

cluster of 16 * 256 uInt8 aname

Especially the last one is not very convinient to do and I personally would treat the entire structure as an array of unsigned bytes passed as a C array pointer and write VIs that insert an retrieve the according information from that byte stream.

I wouild really like to understand the last element, since it gave me the most trouble. I may re-write the dll's handling of data like you suggest, but foresee having to be able to interface with pre-defined dll's that I do not have the source to, so let me take a stab at your example setup:

char aname[16][256]; // LabView: cluster of 16 * 256 uInt8 aname

So you are saying I could define a single cluster with 16*256 = 4096 uInt8 numeric indicators? disregarding the size (which I can see would be *very* inconvenient) does this just have to do with how C stores 2-dimensional char array data? Basically a single block of 8 bit data values representing the 16x256 matrix then. How then would the data be indexed correctly for C? Is it as simple as the value of aname[0][0] as the first block, followed by aname[0][1]...[0][255], then aname[1][0]...[1][255]?

How would char* arr[arrrSize] data (as in a list of pointers to strings) be handled? Would that just be a cluster of arrSize x u8Ints with "Pointer to value" as the "Pass" parameter in the CIN? I suppose I would have to add the NULL byte to the array, so it would actually be a cluster of (arrSize + 1) x u8Ints?

Thanks again, I would really like to understand this correctly so I'm not guessing next time

Another Question: Is this information covered completely in NI's LabWindows course? Or perhaps in the Intermediate or Advanced courses? I'm planning out my training for the year and am trying to prioritize.

BTW, I just tried to create the typedef control with the 4096 indicator cluster and it broke the cluster display in the front panel ;-) Attached.

Download File:post-3266-1150728048.ctl

-- orko

[Rolf Kalbermatter]

I am unfamiliar with what you mean by "inlined". Could you expound on that?

I wouild really like to understand the last element, since it gave me the most trouble. I may re-write the dll's handling of data like you suggest, but foresee having to be able to interface with pre-defined dll's that I do not have the source to, so let me take a stab at your example setup:

Inlined means that a C compiler will simply reserve the necessary amount of bytes for any fixed size data element in the structure itself.

An example

struct {

int val1;

int array[];

}

will represent a structure with 8 bytes length first the val1 integer and then a 32 bit pointer

struct {

int val1;

int array[100];

}

will represent a structure with 404 bytes length, again the val1 integer and the 100 array elements directly following.

Similarily an array

int arr[10][10]

inside a structure will represent a memory area of 400 bytes (10 * 10 * 4 bytes) directly in the structure.

In LabVIEW you would create a cluster of 100 integer elements for this.

An array

int arr[][] would really mean a pointer to an array of pointers instead.

In C multidimensional arrays are a little bit strange if not to say inconsistent. As you can see fixed size arrays look in the organization similar to what the data area of a LabVIEW array does (but a LabVIEW array is always a handle to the data) while non-fixed size multi-dimensional arrays are really hard to deal with.

Element1: I32

Element2: cluster of ( 1D array of U8 with field length set to min length=4, padded with zeros ) (can I just use a string here?)

[\quote]

No a LabVIEW string is always variable sized (except in some embedded development environments) and therefore a handle, so creating a cluster with a string inside will always create a C structure that no normal C code will be compatible with. You could teach your C code to deal with LabVIEW data types directly but that usually requires a good understanding of the LabVIEW datatypes (generally you should have read the External Code Reference Manual from the cover page to the last page ;-) and understand most of it too)

Rolf Kalbermatter