Virtual Instrument

The VI, or Virtual Instrument, is the basic building block of programs written in the G Language. It is similar to a function or subroutine in other programming languages. It includes the Front Panel (containing Controls and Indicators), the Block Diagram (containing Control Terminals, Wires, and Structures and various other GObject nodes), the VI's Icon and its Connector Pane (as well as compiled executable code which is hidden behind the scenes).

The term Virtual Instrument is a play off of the fact that LabVIEW applications are designed for writing software that simulates the functionality of instruments -- rather than being a physical instrument on a laboratory bench, they are virtual and exist in software.

Content of a VI file

By default, a VI contains two kinds of code: the graphical source code that you edit and a compiled version of this code that LabVIEW uses to run the VI.

The graphical source code includes the:

• Front Panel
• Block Diagram
• Data Space
• Connector Pane
• Icon

The compiled version includes the:

• Compiled Code
• Inplaceness Information

When you edit the source code of the VI, LabVIEW automatically recompiles the VI to reflect your changes in the compiled version of the code. LabVIEW also automatically recompiles all VIs that call the changed VI, resulting in unsaved changes to the calling VIs. If the calling VIs are stored in source code control, these pending unsaved changes force you to check out the VIs when you update their subVIs.

To avoid this problem, Starting in LabVIEW 2010, LabVIEW allows you to separate compiled code from the graphical source code of a VI, thereby creating source-only VIs. After you separate the compiled code from a VI, LabVIEW still compiles the VI when you modify subVIs. However, LabVIEW stores the recompiled version of the code in a compiled object cache rather than in the VI in source code control. Because the VI in source code control does not change, you no longer have to check the VI out of source control when you change subVIs.

The VI File Format

VIs are saved as *.vi files, in a proprietary binary format defined by National Instruments, the makers of LabVIEW. The format is strongly inspired by Mac OS Resource Fork format, as LabVIEW was originally created for that platform, and ported to other OSes later, at version 2.5.

The format is a container, storing a group or Resources. Resources have 4-character identifier assigned, and the 4-character identifier for the whole container is RSRC. Actually, most files used by LabVIEW have that RSRC format, only the specific Resources stored inside are different. The files with that header include *.vi, *.ctl, *.mnu, *.llb, *.dlog, *.ctt, *.vit, *.xctl, *.uir, and many other files created with, or internally used by, LabVIEW.

The resources stored within have their identifiers often identical to those in Resource fork format, but many new ones are also introduces. Some resources have multiple variations, as their structure changed between versions of LabVIEW.

A big change in VI format happened in LV version 8.0 - the place where Type descriptors are defined was changed. Before, TDs were defined in-place where they were needed; with 8.0, a new resource VCTP (VI Consolidated Types) was introduced, and all TDs were moved there. Outside VCTP, TDs are now identified by TypeID - index within the consolidated types table.

Known Resources

Below a few resources are listed. Note that the text names given for them are mostly informed guesses.

Resource ID	Long Name	Description
MUID	Map Unique Identifier
FPSE	Front Panel Size Estimate
FPTD	Front Panel Type for DataLog
BDSE	Block Diagram Size Estimate
FLAG	Integer flags
CONP	Connector Port Type Map
CPTM	Connector Port Type Map
CPC2	Connector Port Content Type v2
CPCT	Connector Port Content Type
CPD2	Connector Port DI v2
CPDI	Connector Port DI
DLGH	Dialog HTML
ERRH	Error HTML
HLPT	Help Tag
MItm	M. Item
NODH	NOD HTML
NOEG	NOEG String
STRG	String description
TITL	Title of the file
STR	Short String / Input definition?
CPST	C. P. Strings
DNm#	D. Name Strings List
HDb#	Help Database item
LST#	Short Strings List
STR#	Short Strings List
FDFL	FDFL Strings
LIfp	LinkObj Refs for Front Panel
LIbd	LinkObj Refs for Block diagram
LIds	LinkObj Refs for Data Space
LIvi	LinkObj Refs for VI
DFDS	Default Fill of Data Space
GCDI
BFAL
CGRS	Conglomerate Resource
CPMp	Connection Points Map
DLDR	DL Data Record
DLLP	DL Local Path
FTAB	Font Table
HIST	Changes History
HLPP	Help Path
LPTH	L. Path
HLPW	Help Website URL
SCSR	Syntax Checker Digest
DTHP	Data Types for Heap
DSTM	Data Space Type Map
TM80	Data Space Type Map LV8.0+
LVIN	LabView Instrument
LVSR	LabView Save Record
vers	Version block
PNGI	PNG Image
MNGI	MNG Image
ICON	Icon 32x32 1bpp
ICN#	Icon Large Double 32x64 1bpp
ics#	Icon Small 16x16 1bpp
CURS	Cursor 16x34 1bpp
ics4	Icon Small 16x16 4bpp
icl4	Icon Large 32x32 4bpp
ics8	Icon Small 16x16 8bpp
icl8	Icon Large 32x32 8bpp
BDPW	Block Diagram Password
LIBN	Library Names
LVzp	LabView Zipped Program tree
PRT	Print settings
BNID	B. N. Identifier
CNST	Constant Node UID set
LPIN	Loop Invariant Node UID set
NUID	N. U. Identifier
SUID	S. U. Identifier
VPDP	VI Primitive Dependency Flags
BDHP	Block Diagram Heap
FPHP	Front Panel Heap
BDHb	Block Diagram Heap ver b
FPHb	Front Panel Heap ver b
BDHc	Block Diagram Heap ver c
FPHc	Front Panel Heap ver c
RTSG	Runtime Signature Guid
GCPR	Generated Code Profiler settings
UCRF	Uncompressed Resource File	Used when files are added to RSRC, ie. within *.llb files.
CPRF	'Comp' Compressed Resource File	Used when files are added to RSRC; very basic mask based compression.
ZCRF	ZLib Compressed Resource File	Used when files are added to RSRC; uses compression algorithm from popular Z-Lib library.
DLG3	Dialog Resource File
VCTP	VI Consolidated Data Types	Introduced in LV 8.0, stores all Type descriptors used within the file.
VICD	Virtual Instrument Compiled Data / VI Code	Contains the machine code generated by LLVM compiler.
VITS	Virtual Instrument Tag Strings

See Also

• Separating Compiled Code
• NI LabVIEW Compiler