These are the methods that device drivers provide for functions that can't be accomplished through normal i/o. For example, a tape driver will have an ioctl call to set compression. See https://aplawrence.com/Unix/lpstatus.html for more examples.
On more modern systems, these are deprecated and replaced with things like Linux /proc and (2.6) /sys pseudo filesystems.
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More Articles by Tony Lawrence © 2011-03-20 Tony Lawrence
Computers have been taught to distrust each other and will reject attempted connections most of the time. Nowadays, most computers and firewalls are utterly rude about it: it would be like asking someone to dance and having them ignore you as though you were invisible and inaudible. (Tony Lawrence)