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$IO

Contains the ID of the current input/output device.

Synopsis

$IO
$I

Description

$IO contains the device ID of the current device to which all input/output operations are directed. If the input and output devices are different, $IO contains the ID of the current input device.

InterSystems IRIS sets the value of $IO to the principal input/output device at login. $PRINCIPAL contains the ID of the principal device. You issue a USE command to change the current device. Only the USE and CLOSE commands, a BREAK command, or a return to the Terminal can change this value.

You can return the device type of the current device by using the GetType() method of the %Library.Device class.

On UNIX® systems, $IO contains the actual device name.

On Windows systems, $IO contains an InterSystems IRIS-generated unique identifier for the principal device. For terminal devices (TRM or TNT), this consists of a pseudo-device name enclosed in vertical bars, a colon and another vertical bar, followed by the device’s process ID (pid) number. For non-terminal devices, the pseudo-device name is enclosed in vertical bars and followed by a unique numeric identifier.

For a Terminal: |TRM|:|pid

For a Telnet terminal: |TNT|nodename:portnumber|pid

For a file descriptor: |FD|file_descriptor_number

(File descriptors are used with CALLIN/CALLOUT remote access.)

For a TCP device: |TCP|unique_device_identifier

For a named pipe: |NPIPE|unique_device_identifier

For the default printer: |PRN|

For a printer other than the default: |PRN|physical_device_name

If the principal device is a null device (which is the default for a background process), $IO contains the null device name with ":pid" appended, thus allowing you to use $IO for a unique subscript. The null device name contained in $IO depends on the operating system.

  • For Windows systems, $IO contains //./nul:pid

  • For UNIX® systems, $IO contains /dev/null:pid

If the input device is redirected via a pipe or file, $IO contains “00”.

The default device number for a device is configurable. Go to the Management Portal, select System Administration, Configuration, Device Settings, Devices. For the desired device, click “Edit” to display and modify its Physical Device Name: option. If you do this, $IO will contain the assigned device number, rather than the actual operating system device name.

This special variable cannot be modified using the SET command. Attempting to do so results in a <SYNTAX> error.

See Also