Open62541 - read simple UA_String attribute

[levon....@g-mail.fr]

Hello !

I'm tying to create a OPC UA Client using open62541  implementation of OPC UA with Qt5.

As I am a beginner I have trouble to read simple String on the server and cast it to QString so i can display it ...

I have created a custom function  'readStringValue()'  :

UA_StatusCode MyUaClient::readStringValue(UA_UInt16 nsIndex, QString nodeId)
{
               
            UA_StatusCode retval;
            UA_NodeId node = UA_NODEID_STRING_ALLOC(nsIndex, nodeId.toLatin1().data());

             UA_Variant value;
            // UA_Variant_init(&value);


             UA_Variant *val = UA_Variant_new();
                 UA_StatusCode retval;
               retval = UA_Client_readValueAttribute(client,node, val);


               if(retval == UA_STATUSCODE_GOOD  &&
                  val->type == &UA_TYPES[UA_TYPES_STRING]) {

                 value = *(UA_Variant*)val->data;


                       printf("the new value is: %i\n", value);
               }
           UA_Variant_delete(val);
            return retval;
}


On the server my "PLC_IO_String_1" value is 'test' and type is String

But when i call my function 'readStringValue' :  readStringValue(1,"PLC_IO_String_1"); 

it returns :  the new value is : 4

So i think i have data type conversion probleme. 

Does anyone know where the problem comes from and how to fix this please ?

Thank you in advance 

LA 

[Stefan Profanter]

Hi,
by quickly looking at your code I think there are 3 problems:

1. Change value to be of type String:
UA_String value;

2. The cast should be a string (since you are checking it in the if before):
value = *(UA_String*)val->data;

3. Your printf uses '%i' as the format which cannot be used for strings. It needs to be %s in general. Since you try to output a UA_String, you need to explicitly give the length of the string you want to print. E.g.
printf("New value: %.*s", value.lenght, value.data);

Also note that if you call UA_Variant_delete on val, it may also delete the content of value. Thus it's better to use UA_String_copy for point 2 or create a QtString in that line.

Hope this helps?
Stefan

[basyskom.fran...@gmail.com]

I would like to encourage you to have a look at the QtOpcua module.

http://code.qt.io/cgit/qt/qtopcua.git/tree/

It will offer you a default translation of OPC-UA types into Qt types.

It also implements an asynchronous API which prevents you from blocking your Qt event loop.

Cheers,

Frank

Am Mittwoch, 4. Oktober 2017 16:55:15 UTC+2 schrieb LEVON AGAYEV:

[LEVON AGAYEV]

Hello Frank,

Is there a documentation on how can i use QtOpcua module ?
Do i have to compile your project ? or is there a .h/.cpp to include just like open62541 ?

It is failing to clone your project from http://code.qt.io/cgit/qt/qtopcua.git/tree/ do you know why ?

Thank you very much,

LA 

[basyskom.fran...@gmail.com]

Hello Levon,

QtOpcua will be part of the next official Qt release (5.11) as a Tech-Preview (aka. a module which doesn't provide API and ABI stability yet).

Given the current release plan (https://wiki.qt.io/Qt_5.11_Release) alpha builds would be available end of February.

Till then you have to compile the "dev" branch of Qt yourself.

Which platform are you on?

Kind Regards,

Frank