2.2 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 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.

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.