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Low-Level File I/O VI and Functions
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:
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:
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:
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:
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
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.
 |
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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This work is licensed by National Instruments under a Creative Commons Attribution License (CC-BY 1.0), and is an Open Educational Resource.
Last edited by Elizabeth Gregory on Jul 16, 2004 3:25 pm GMT-5.
"A full introductory course on programming with LabVIEW."