Channel Command Component

See Also: File I/O Commands

Purpose

To enable concurrent opening of more than one output destination or input source, and to enable switching from one channel to another as an application executes.

Syntax

Channel {channelNumber}

Where {channelNumber} is the number of the channel.

Values

• 0 - 9

What It Does

The channel command component is an option in the following commands:

• append_output
• close_input
• close_output
• direct_input
• direct_output
• read
• read_block
• readln
• write
• Writeln

Use the New_Seq_Channel function to obtain a new channel, and the Seq_Release_Channel function to release a channel that is no longer used. See the section Sequential File Input and Output to read more about file input and output (I/O) in DataFlex.

It is recommended to always use channels when doing any file I/O to make your code more reusable.

Example

Use seq_chnl.pkg

Procedure WriteTwoFiles
    Integer iChOut1 iChOut2
    String sFileOut1 sFileOut2

    // specify the file name of the first output file
    Move "c:\temp\OutFile1.txt" to sFileOut1

    // specify the file name of the second output file
    Move "c:\temp\OutFile2.txt" to sFileOut2

    // obtain 2 open channels
    Move (Seq_New_Channel()) to iChOut1
    Move (Seq_New_Channel()) to iChOut2

    Direct_output channel iChOut1 sFileOut1  // open output channel
    Direct_output channel iChOut2 sFileOut2  // open output channel

    Writeln channel iChOut1 "This is line 1 of output file 1"
    Writeln channel iChOut2 "This is line 1 of output file 2"
    Writeln channel iChOut1 "This is line 2 of output file 1"
    Writeln channel iChOut2 "This is line 2 of output file 2"
    Writeln channel iChOut1 "This is line 3 of output file 1"
    Writeln channel iChOut2 "This is line 3 of output file 2"

    Close_Output channel iChOut1  // close output channel
    Close_Output channel iChOut2  // close output channel

    // release both channels for reuse
    Send Seq_Release_Channel iChOut1
    Send Seq_Release_Channel iChOut2
End_Procedure

This example writes 3 lines to 2 different output files. Using the channel command component, the code is able to alternate writing between the 2 files.

Notes

Whenever a channel is specified and subsequent lines of code do any I/O without specifying a channel, the last specified channel will be used. This is a common cause of coding errors.

Example

Writeln channel iChOut1 "This is line 1 of output file 1"
Writeln "This is line 1 of output file 2"

If the above change was made to the example in the What It Does section, both lines would be written to the file open in channel iChOut1.