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 afile dialog box. You can optionally specify a dialog prompt, default file name, start path, or filter pattern. If filepath 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 alocation 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 offsetfor byte stream files and at the end of the file for datalog files. data, header, and the format of the specified filedetermine 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 operatesuniquely in this function, which closes regardless of whether an error occurred in a preceding operation. Thisensures 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 ofthe 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 inand 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 applicationsneed 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 dialogbox, do not wire
file path
in the
Open/Create/Replace File
VI. However, you can
save time by programmatically wiring the default filename andpath to the VI.
describes how pathnames
are organized.
Platform | Pathname |
---|---|
Windows | Consists of the drive name, a colon,
backslash-separated directory names, and thefilename. 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 thefilename. 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 shows how to write string data to a file while programmatically wiring thefilename 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 onopen files, devices, or network connections use
refnum
s to identify each object.
The error cluster and
refnum
pass in
sequence from one node to the next. Because a node cannotexecute until it receives all its inputs, passing these two
parameters forces the nodes to run in order and creates a datadependency. 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 theerror cluster to the
Simple Error Handler
VI.