To allow GW-BASIC to send a "control data" string to a character device driver anytime after the driver has been opened.
file number is the file number open to the device driver.
string is a valid string expression containing characters that control the device.
IOCTL commands are generally 2 to 3 characters followed by an optional alphanumeric argument. An IOCTL string may be up to 255 bytes long, with commands within the string separated by semicolons.
If a user had installed a driver to replace lpt1, and that driver was able to set page length (the number of lines to print on a page before issuing a form feed), then the following lines would open the new lpt1 driver and set the page length to 66 lines:
OPEN "LPT1:" FOR OUTPUT AS #1 IOCTL #1, "PL66"
The following statements open lpt1 with an initial page length of 56 lines:
OPEN "\DEV\LPT1" FOR OUTPUT AS #1 IOCTL #1, "PL56"