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From: amoss@huji.ac.il (Amos Shapira)
Newsgroups: comp.lang.c
Subject: Re: How do you tell if stdin/stdout is a terminal?
Keywords: stdin/stdout, unix, C, redirected I/O
Message-ID: <amoss.649953828@mush>
Date: 6 Aug 90 14:43:48 GMT
References: <12210@hydra.gatech.EDU>
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 22

Hello,

gg10@prism.gatech.EDU (GALLOWAY) writes:

>So, how do they do it on a generic unix box?

Simple, don't know how much compatible it is, but there should be a routine
in the standard C library called "isatty(fd)" which will return 1 if fd
(a file descriptor) is a terminal and 0 otherwise.

If you don't have isatty(), it's way of operation is simple, it just does
an ioctl that only a terminal will accept without an error, like
"ioctl(fd, TIOGETP, &junk)" and if it returns a value less than a 0 then
fd is not a tty, otherwise it is. Not that it does a "GET*" operation, so
it doesn't brake anything.

Just for the sake of completeness, to convert from a file pointer (FILE *)
to a file descriptor (int), use fileno(fp).

Hope this helps,
Amos Shapira.
amoss@batata.huji.ac.il