Path: utzoo!attcan!uunet!mailrus!iuvax!maytag!watstat!dmurdoch
From: dmurdoch@watstat.uwaterloo.ca (Duncan Murdoch)
Newsgroups: comp.os.msdos.programmer
Subject: Re: How to detect redirection?
Message-ID: <1990Aug8.134837.4637@maytag.waterloo.edu>
Date: 8 Aug 90 13:48:37 GMT
References: <1426@wet.UUCP> <1990Aug8.004304.26989@Octopus.COM>
Sender: daemon@maytag.waterloo.edu (Admin)
Organization: University of Waterloo
Lines: 35

In article <1990Aug8.004304.26989@Octopus.COM> stever@octopus.UUCP (Steve Resnick ) writes:
>that character may already be on the screen. In C the easiest way to determine
>if I/O has been redirected is to call isatty(fileno(stdout)) this returns
>non-zero if the file descriptor associated with stdout is a character device.
>The problem with this is the redirection may be associated with another
>character device (AUX, PRN, etc). Is there an easy way to do THIS?

Use the IOCTL service from DOS.  To quote Ralf Brown's interrupt list:

INT 21 - DOS 2+ - IOCTL - GET DEVICE INFORMATION
        AX = 4400h
        BX = file or device handle
Return: CF set on error
           AX = error code (see AH=59h)
        CF clear if successful
           DX = device info
           If bit 7 set: (character device)
                   1: console output device
                   2: NUL device
                   3: CLOCK$ device
                   4: device is special (uses INT 29)
                   5: binary (raw) mode
                   6: Not EOF
                  12: network device (DOS 3+)
                  14: can process IOCTL control strings (see AL = 02h-05h)
           If bit 7 clear: (file)
               bits 0-5 are block device number
                   6: file has not been written
                  12: network device (DOS 3+)
                  14: can process IOCTL control strings (DOS 3+)
                  15: file is remote (DOS 3+)
SeeAlso: AX=4401h, INT 2F/AX=122Bh


Duncan Murdoch