lpr output filters

[Lloyd Kremer]

In article <8185 at chinet.chi.il.us> les at chinet.chi.il.us (Leslie Mikesell)
writes:

>*In article <1159 at novavax.UUCP> gls at novavax.UUCP (Gary Schaps) writes:
>*>> > Does it seem reasonable to try to create an lpr output filter which 
>*>> > selectively prevents certain files from being printed?
>*
>*>I would like to "protect" certain files by making them "view only". 
>
>...................................  The best you could do would be
>to make the files unreadable by normal users and provide a setuid program
>that displays the contents only if the output is directly to a terminal.

Even this can be tricky to get right in all cases.  I just ran this program
on our AT&T 3B2 running UNIX(tm) System V Release 3:

	#include <fcntl.h>

	main()
	{
		int fd;

		if((fd = open("/dev/tty15", O_WRONLY)) != -1){
			printf("isatty() returns %d\n", isatty(fd));
			close(fd);
		}
		return(0);
	}

It produced:

	isatty() returns 1

On our system, /dev/tty15 is a NEC Pinwriter P2 parallel printer.
Not all "ttys" are real terminals.

					Lloyd Kremer
					Brooks Financial Systems
					{uunet,sun,...}!xanth!brooks!lloyd