Hi, I have a LabVIEW program that communicates via TCP/IP with an electronic device. The messages exchanged are binary and serialized. To deserialize them, I call a Qt DLL that takes the encoded message as input and returns the decoded message.In Qt, we use a QByteArray for binary messages. However, LabVIEW does not know this type of data. So I decided to exchange strings ( Qt and LabVIEW know how to convert byte arrays to strings).The problem is that the CLFN does not return any data. Here is more information : I have to decodedMsg : 1 in string, 1 in hex. Here myDLL.dll : std::string* myDLL::decodeMsg(std::string* receivedMessageString) { QByteArray receivedMessage(receivedMessageString->c_str(), receivedMessageString->length()); // Deserialize std::string response(receivedMessage.constData(), receivedMessage.length()); return &response; Just simply DLL that return what I send. The response : CLFN configuration : I don't understand why I can't get a response. Thanks for help.

@syuiopwdfghj wrote: I already pass a CStr to my DLL. That is not what you showed in your code. I could pass an unsigned char array to my DLL (to an unsigned char *) but I could not receive such an array because Labview can only retrieve String and Numerics. LabVIEW strings are internally unsigned char arrays but wrapped inside a handle. But if your DLL says that it receives char* or unsigned char *, doesn't really matter other than an interpretational difference. So a possible way to solve your problem is like this: int decodeMsg((unsigned) char *receivedString, (unsigned) char *responseString, size_t *size) { QByteArray receivedMessage(receivedString, strlen(receivedString); // Deserialize std::string response(receivedMessage.constData(), receivedMessage.length()); if (*size <= response.length()) return buffToSmall; strncpy(responseString, response.c_str(), response.length() + 1); *size = response.length(); return noError; Most likely you can also get rid of the creation of the response std:string variable by directly mem-copying the bytes from the QByteArray const buffer into the C string buffer and appending an explicit 0 termination character yourself to the string. A very fancy but LabVIEW specific function (that requires to also link your DLL to labviewv.lib in your cintools directory and include at least extcode.h from the same directory) would be this: MgErr decodeMsg(LStrHandle receivedString, LStrHandle *responseString) { QByteArray receivedMessage(LStrBuf(*receivedString), LStrLen(receivedString); // Deserialize MgErr err = NumericArrayResize(uB, 1, (UHandle*)responseString, receivedMessage.length()); if (!err) { MoveBlock(receivedMessage.constData(), LStrBuf(**responseString), receivedMessage.length()); LStrLen(**responseString) = receivedMessage.length(); } return err; }

LabVIEW

Use Qt DLL into Labview

Solved!

Still not enough information. What is the size of the encoded Message string?

Even if it is configured to be encodedMessageSize, are you sure the qt generated string is ALWAYS smaller than the length of the passed in string?

If you call C functions you have to make sure that the passed in memory buffers that you try to have your DLL write data into is always AT least as large preallocated as the DLL function tries to write into. That's the only safe method if you have unmanaged data buffers. And if you use managed data buffers, the consumer (your DLL) needs to use the management contract of the caller. For a LabVIEW called DLL that is the LabVIEW memory managed data handles for arrays and strings!

Rolf Kalbermatter
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