How to determine string length when dereferencing string pointer using LabVIEW MoveBlock

[SteveChandler]

Hello!

I am calling a DLL which returns a pointer to a structure consisting of a pointer to a string, a 32 bit integer, and another pointer to another string.

I have managed to use MoveBlock to return a cluster of these three things and that works fine.

But now I need to read the two strings that I have pointers to which are not fixed length. MoveBlock has a size input but how do I determine how long the strings are?

[Darin.K · Accepted solution]

Try this:

...\vi.lib\Utility\importsl\GetValueByPointer\GetValueByPointer.xnode

Just wire a string constant to the Input Type.  If I recall correctly the terminating null (assuming C strings) will be included.

Sometimes I just MoveBlock one byte at a time until I reach a null or a specified maximum size.

[Jacobson-ni]

Steve,

You can use GetValueByPointer without needed to know the length of the string before hand.  The community example for dereferencing pointers mentions that this is the perferred method for strings

<LabVIEW>\vi.lib\Utility\importsl\GetValueByPointer\GetValueByPointer.xnode

[SteveChandler]

Thanks. I tried GetValueByPointer but it would not work in an exe. I got an error of some kind. I think it was an error 7 file not found. I can revert the code back to when I was using that and check again when I have access.

As for reading 1 byte at a time, do I just read until I get a 0 or whatever the NULL is in C? Also would performance take a hit?

[SteveChandler]

Thanks Darin. You gave two valid solutions.

The one that I am going with is reading a byte at a time until I reach the NULL. I ran into a separate problem using the xnode when running as an exe. I probably won't have time to investigate that further but it was something about file not found.

I am glad to see that all my favorite avatar's are still here answering questions! I have been really busy doing non-LabVIEW things so I have not had as much time for my favorite forum.

[smithd · Accepted solution]

If you configure a CLFN to call StrLen (http://zone.ni.com/reference/en-XX/help/371361K-01/lvexcode/strlen/) you should be able to get that info. Depending on the format there are many other support functions you can call that are more well documented (http://zone.ni.com/reference/en-XX/help/371361K-01/lvexcode/support_manager_functions/) and less well documented (<LabVIEW>/cintools/extcode.h). If I'm not mistaken you should be able to call anything in extcode.h (except the macros of course) from a CLFN, just like you're using moveblock.

[SteveChandler]

Awesome, thanks. I could not get this to work when I had MoveBlock configured with destination as a string. It read more than it should have and caused LabVIEW to crash. I probably had it configured wrong but what I ended up doing was to initialize a U8 array to the size returned by StrLen then use MoveBlock to fill this array, and convert the array to a string.