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:
The compiled version includes the:
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 introduced. 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 | Stores a bitmap picture in PNG format. |
| MNGI | MNG Image | While PNGs become quite popular, their animated counterpart, MNG format, never really caught on. |
| ICON | Icon 32x32 1bpp | |
| ICN# | Icon Large Double 32x64 1bpp | Usually used to store one icon in two versions (ie. disabled and active). |
| ics# | Icon Small 16x16 1bpp | |
| CURS | Cursor 16x34 1bpp | Usually used to store one cursor in two versions (ie. disabled and active). |
| 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 | A whole application compressed to ZIP file. |
| 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 |