Problem writing string

[Zeno82]

Hi everybody,

I have read/write access to a string variable. It's a VT_BSTR.
Reading works fine, I have issues with writing.

First question is: how do I know how many charachters (or bytes) I'm allowed to write, if I didn't get any specs in advance?

Second: How do I make use of all the space I have available?
I ask this because when the string holds, e.g., 10 wchars, I can only write 10 or less.
Using Matrikon OPC I can write all the characters number the PLC developer told me.
My code so far:

std::wstring str(L"The string I want to write");
OPCItemData data;
var.pItem->readSync(data, OPC_DS_DEVICE);
wcscpy_s(data.vDataValue.bstrVal, 64, str.c_str()); // I know I have 64 wchars available
var.pItem->writeSync(data.vDataValue);

"var" is a struct I created:

struct VARIABILE  
{  
	std::string Nome;  
	COPCItem* pItem;  
	VARTYPE tipo;  
};

The above code works well only if the string already holds more than str.length(), otherwise the string gets truncated (and I fear I might be overwriting memory areas somewhere).

Any help is appreciated.