LabVIEW
Need help with the Call Llibrary Function Node when a DLL returns a pointer to data buffer.
poethomer wrote:Syntax
SSIDLL_API int __stdcall SetDecodeBuffer(int nComPort, unsigned char *pData, long
max_length);
where:
. nComPort is the COM port number used in the call to SSIConnect
. pData is a pointer to the destination buffer for decode data returned from the
scanner
. max_length is the size in bytes of the destination buffer
If this prototype is correct, the DLL function does NOT return a pointer. Instead it expects to be passed a memory pointer by the calling application, that is presumably supposed to be at least max_length bytes long. There is no other way!
However the name of the function gives reasonable support to the assumption that it is just assigning a string pointer to some internal global variable that the DLL will then fill in AFTER that function has returned. This is a way of operation that is very special and assumes that the buffer that was passed to SetDecodeBuffer() stays valid AFTER the control returns back to LabVIEW from that function.This is not generally the case for LabVIEW variables that reside in a wire. Once a wire stops existing on the diagram, LabVIEW not only reserves the right to reuse its buffer but in general also does so. So you would have to make sure that the wire stays in the diagram without further wire branches until after you can be sure the buffer has been filled and after you have used the aforementioned memory copy function to copy out the now available information from it.
It is uttermost important to not branch the wire on its way to the copy memory function in any way, as this would cause LabVIEW to most likely create a copy of the memory buffer and it would be sheer luck if the original buffer is passed to you copy memory function or the other place you were branching of to.
Remember the second section is entirely based on assumptions because of the function name. What this function really does and how it is supposed to be used should be explained in detail in the according documentation to the DLL. If it isn't, that is a problem of the DLL creator or distributor, not LabVIEW.
