topic Re: Calling a C Struct from a LV CallLibrary Function. in LabVIEW

Calling a C Struct from a LV CallLibrary Function.

MogaRaghu — Tue, 19 Jul 2011 12:27:15 GMT

I have a DLL called Canopen.dll.

I have an example C-Code on how to use the functions in the DLL.

Have been able to call a few of the functions in LV using the Call Library Function.

But one function is a problem as its arguments are from a Struct. It goes like this:

Function Call
short CMA_ConfigCANChannel(
unsigned char usChannelHandle,
CMA_CAN_CONFIG stCANConfig);

Parameter List

typedef struct
{
unsigned char ucBaudrate;
unsigned char ucPresc;
unsigned char ucSjw;
unsigned char ucTSeg1;
unsigned char ucTSeg2;
unsigned char ucAccCode;
unsigned char ucAccMask;
} CMA_CAN_CONFIG;

Now how to convert this to be suitable fro CallLibrary Function ?? ( Actually there is one example in the NI site but that is in LV5.0 and my LV2009 cannot open it !!)

Thanks

Re: Calling a C Struct from a LV CallLibrary Function.

smercurio_fc — Tue, 19 Jul 2011 14:52:26 GMT

Create a cluster consisting of U8 integers. You will need to use 8 of them since the struct has 7 and you will need to make sure you account for alignment to memory locations. You can then just use "Adapt to Type" for the parameter type. LabVIEW comes with a very detailed example on calling DLLs. Open the Example Finder and search for "DLL".

Re : Calling a C Struct from a LV CallLibrary Function.

Julien_31 — Tue, 19 Jul 2011 14:52:53 GMT

Hi,

I'm not sure but I suggest you use a cluster (a LabVIEW Cluster is equivalent to a C Struct).

So in your case, you may pass a 7 item cluster to your "call library function node" (cluster containing ucBaudrate, ucPresc, etc...).

Hope this helps.

Best,

Re : Calling a C Struct from a LV CallLibrary Function.

Julien_31 — Tue, 19 Jul 2011 14:54:22 GMT

Oups, smercurio answered before I post my answer. I think it's explanation is most complete than mine. 🙂

Re : Calling a C Struct from a LV CallLibrary Function.

nathand — Tue, 19 Jul 2011 18:46:53 GMT

Unfortunately I think smercurio's answer is incorrect in this case. Rolfk answered this question here. To pass a struct by value, create one parameter for each element of the struct.

Re : Calling a C Struct from a LV CallLibrary Function.

smercurio_fc — Tue, 19 Jul 2011 19:08:55 GMT

You know, I didn't even see it was being passed by value. Guess it's habit for me to always see it passed by reference.

Re: Calling a C Struct from a LV CallLibrary Function.

Kousy — Mon, 25 Jul 2011 10:19:20 GMT

How to handle int array and string array inside a C structure to call it from LV call library function?

Function:

short CMA_GetVersion( unsigned char ucChannelHandle, CMA_VersionStruct *stpVersion);

C Structure:
typedef struct
{
unsigned long ulHWSerialNumber;
unsigned char ucHWMainRevision;
unsigned char ucHWSubRevision;
unsigned char ucHWIdentString[18];
unsigned char ucSWMajorVersion;
unsigned char ucSWMinorVersion;
unsigned char ucSWBuild;
} CMA_VersionStruct;

Function:

short CMA_InstallPDO_E( unsigned char ucChannelHandle, CMA_PDOConfig, unsigned short *uspPDOHandle);

C Structure:
typedef struct
{
unsigned char ucPDOType;
unsigned long ulCOBID;
unsigned char ucTransType;
unsigned char ucAccType;
unsigned char ucEventNotif;
unsigned char ucaDefVal[8];
unsigned short usEventTimer;

unsigned char ucNrOfByte;

unsigned char ucNoInitTrans;
} CMA_PDOConfig;

Hope someone helps.

Thanks

Kousy

Re: Calling a C Struct from a LV CallLibrary Function.

nathand — Mon, 25 Jul 2011 16:47:16 GMT

This has been asked over and over again on this forum. You need to replace the array in the C struct with a LabVIEW cluster containing the same number of elements. For example, you would replace ucHWIdentString[18] with a cluster of 18 U8 values. The easiest way to generate such a cluster is by wiring an empty array of U8 to "Array to Cluster" and setting the number of cluster elements appropriately. Create a control or constant as necessary from the output of Array to Cluster. To get a string into that structure, you can use String to Array of U8, followed by Array to Cluster. The reverse operation will get the you the string out of that structure.

Re: Calling a C Struct from a LV CallLibrary Function.

Kousy — Wed, 27 Jul 2011 08:53:36 GMT

nathand,

I have tried Adapt to Type and Handle by value for the above mentioned functions. Still Im getting LV Error 1097 when trying out GetVersion function call and Functional Error of -110 when trying out InstallPDO_E. Im attaching the VI. Can you please look into it?

Thanks

Kousy

Re: Calling a C Struct from a LV CallLibrary Function.

nathand — Thu, 28 Jul 2011 00:29:55 GMT

Based on the random clusters floating around your block diagram, it looks like you tried everything except the one thing I suggested. Create a cluster that matches your C structure, and replace the fixed-size arrays with equivalent clusters. For the CMA_VersionStruct, that means that ucHWIdentString should be replaced with a cluster containing 18 U8 values (U8 is the same size as char). Then pass the entire CMA_VersionStruct cluster as a single parameter set to Adapt to Type (the Data Format doesn't matter), because you want to pass it by reference, not by value.

Re: Calling a C Struct from a LV CallLibrary Function.

Kousy — Thu, 28 Jul 2011 11:27:17 GMT

Thanks nathand.

Now the function call CMA_GetVersion is working. But the other function CMA_InstallPDO_E still throws a fault. Herewith Im attaching the debugging of dll's snapshot for reference with COBID of 202, as you can see the COBID entry is garbled. Could you please look into it and clarify where the code is went wrong?

Thanks

Kousy

Re: Calling a C Struct from a LV CallLibrary Function.

smercurio_fc — Thu, 28 Jul 2011 13:20:10 GMT

ucaDefVal is defined as an array of 8 char values, and you are passing in an array of 9 elements.

Re: Calling a C Struct from a LV CallLibrary Function.

nathand — Thu, 28 Jul 2011 15:52:59 GMT

@smercurio_fc wrote:

ucaDefVal is defined as an array of 8 char values, and you are passing in an array of 9 elements.

I'm certain that this is not the problem. LabVIEW will pass a native array by pointer, so the number of elements in it is irrelevant. The C struct defines a fixed-size array and it is being passed by value. Using a LabVIEW array is not an option at all, regardless of the number of elements in it.

Re: Calling a C Struct from a LV CallLibrary Function.

smercurio_fc — Thu, 28 Jul 2011 16:20:24 GMT

I've got to stop jumping back and forth between LabVIEW, Excel, Matlab, C#, C, etc.. My brain is officially liquified this morning.

Re: Calling a C Struct from a LV CallLibrary Function.

nathand — Fri, 29 Jul 2011 20:25:50 GMT

Kousy wrote:

Now the function call CMA_GetVersion is working. But the other function CMA_InstallPDO_E still throws a fault. Herewith Im attaching the debugging of dll's snapshot for reference with COBID of 202, as you can see the COBID entry is garbled. Could you please look into it and clarify where the code is went wrong?

It looks to me like you need to pass an additional 3 bytes of padding between ucPDOType and ulCOBID to maintain alignment. This would be necessary if you were passing the cluster by reference so I wouldn't be surprised if it's required for passing by value as well. The reason is that in C at least, a long needs to be aligned on a 4-byte boundary, whereas LabVIEW always packs structures as tightly as possible. The first value in the cluster, ucPDOType, will be properly aligned because it starts the structure. Another 3 bytes are then necessary to reach the next 4-byte boundary on which the unsigned long ulCOBID can start. You can just add two dummy parameters - a U8 and a U16 - and pass null values to them.

You also need to pass 8 separate U8 (char) values for ucaDefVal (again, due to passing by value) instead of passing an array by reference.

Re: Calling a C Struct from a LV CallLibrary Function.

MogaRaghu — Sat, 30 Jul 2011 12:14:21 GMT

Nathand !

Thanks for your continued support. We have tried all methods as suggested and still the CMA_Install PDO_E throws the same fault. While reading the device manual this is what it says about calling functions from Labview :

The parameter structures must be defined as arrays and need

to be evaluated by bytes. The array size must fit or exceed the

size of the parameter structure. Otherwise, access violations

during operation may occur.

Does this throw any additonal light on the problem ??

Re: Calling a C Struct from a LV CallLibrary Function.

nathand — Mon, 01 Aug 2011 02:14:07 GMT

I'm not sure exactly what you tried, so I'm posting what I'm pretty sure it should look like:

Note the addition of 4 bytes of padding: 3 bytes (one U8 and one U16) before ulCOBID, and 1 byte before usEventTimer. The snippet creation seems to mess up cluster images; the ucaDefVal cluster is of course 8 U8 elements.

If this is what you already tried, and you're still getting an error, please specify exactly what the error is and whether it's an error from LabVIEW or from your DLL. Also, in the Call Library Node, try setting debugging to maximum.

Are you and Kousy working on the same team, or just coincidentally trying to use the same DLL at the same time?

Re: Calling a C Struct from a LV CallLibrary Function.

RavensFan — Mon, 01 Aug 2011 02:41:29 GMT

Nathan,

Don't use the snippeet creator built into LabVIEW. Download the Code Capture Tool. It works much better. It also doesn't break property nodes the way the LabVIEW version does.

Community nugget: Code Capture Tool 2.0

Re: Calling a C Struct from a LV CallLibrary Function.

Kousy — Mon, 01 Aug 2011 10:41:04 GMT

Nathand,

Tried as you suggested. Its not working out. And Im getting the same dll error.

Just to make matter simple we are trying another functionality called ConfigSyncMan. We tried to send the syncID(0x80) as simple U32 datatype as well as in both endian formats , neither worked out.

Raghunathan and self are working on the same project.

What else can I try?

Thanks.

Kousy

Re: Calling a C Struct from a LV CallLibrary Function.

nathand — Mon, 01 Aug 2011 16:33:36 GMT

@Kousy wrote:

Tried as you suggested. Its not working out. And Im getting the same dll error.

Could you be more specific - is the error that you're getting a LabVIEW error, or an error from the DLL? If from LabVIEW, is it 1097 or something else? If from the DLL, consult the DLL documentation for the meaning of the error code - I can't help debug that.

If you run the Install_PDO_E call exactly as I suggested and then watch the call in the debugger, like the screenshot you posted, do the right values go to the right places?

What is the prototype of the ConfigSyncMan function? If this is another case of passing a struct by value, then you probably need two bytes of padding to align ulSyncID. There is no need to flatten and unflatten your parameters - LabVIEW will take care of the endianness.