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Low-Level File I/O VI and Functions

Module by: National Instruments

Summary: In this lesson, you will learn how to use the low-level File I/O VI and functions.

Use the following low-level File I/O VI and functions to perform basic file I/O operations:
Open/Create/Replace File -  OpenFileIcon.png Opens an existing file, creates a new file, or replaces an existing file, programmatically or interactively using a file dialog box. You can optionally specify a dialog prompt, default file name, start path, or filter pattern. If file path is empty, the VI displays a dialog box from which you can select a file.
Read File -  ReadFileIcon.png Reads data from an open file specified by refnum and returns it in data. Reading begins at the current file mark or a location specified by pos mode and pos offset. How the data is read depends on the format of the specified file.
Write File -  WriteFileIcon.png Writes data to an open file specified by refnum. Writing begins at a location specified by pos mode and pos offset for byte stream files and at the end of the file for datalog files. data, header, and the format of the specified file determine the amount of data written.
Close File -  CloseFileIcon.png Closes an open file specified by refnum and returns the path to the file associated with the refnum. Error I/O operates uniquely in this function, which closes regardless of whether an error occurred in a preceding operation. This ensures that files are closed correctly.

Error Handling

Error.png
The low-level File I/O VIs and functions return error information. Wire the error information from the beginning of the VI to the end. Include an error handler VI, such as the Simple Error Handler VI located on the Time & Dialog palette, at the end of the VI to determine if the VI ran without errors. Use the error in and error out clusters in each VI you use or build to pass the error information through the VI.

Saving Data in a New or Existing File

You can write any data type to a file you open or create with the File I/O VIs and functions. If other users or applications need to access the file, write string data in ASCII format to the file. Refer to the LabVIEW Basics II: Development Course Manual for more information about writing LabVIEW datalog or binary files.
You can access files either programmatically or interactively through a file dialog box. To access a file through a dialog box, do not wire file path in the Open/Create/Replace File VI. However, you can save time by programmatically wiring the default filename and path to the VI. Table 1 describes how pathnames are organized.
Platform Pathname
Windows Consists of the drive name, a colon, backslash-separated directory names, and the filename. For example, c:\testdata\test1.dat is the pathname to a file named test1.dat in the testdata directory.
UNIX UNIX Consists of forward slash-separated directory names and the filename. For example, /home/testdata/test1.dat is the pathname to a file named test1.dat in the testdata directory in the /home directory. Filenames and pathnames are case sensitive.
Mac OS Consists of the volume name (the name of the disk), a colon, colon-separated folder names, and the filename. For example, Hard Disk:testdata:test1.dat is the pathname to a file named test1.dat in a folder named testdata on a disk named Hard Disk.
The block diagram shown in Figure 1 shows how to write string data to a file while programmatically wiring the filename and pathname. If the file already exists, it is replaced; otherwise a new file is created.
pathnameex.png
Figure 1
The Open/Create/Replace File VI opens the file test1.dat. The VI also generates a refnum and an error cluster.
When you open a file, device, or network connection, LabVIEW creates a refnum associated with that file, device, or network connection. All operations you perform on open files, devices, or network connections use refnums to identify each object.
The error cluster and refnum pass in sequence from one node to the next. Because a node cannot execute until it receives all its inputs, passing these two parameters forces the nodes to run in order and creates a data dependency. The Open/Create/Replace File VI passes the refnum and error cluster to the Write File function, which writes the data to disk. When the Write File function finishes execution, it passes the refnum and error cluster to the Close File function, which closes the file. The Simple Error Handler VI examines the error cluster and displays a dialog box if an error occurred. If an error occurs in one node, subsequent nodes do not execute, and the VI passes the error cluster to the Simple Error Handler VI.

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