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Re: Help with pipes, buffering and pseudoterminals

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  Re: Help with pipes, buffering and pseudoterminals Chris Angelico <rosuav@gmail.com> - 2015-04-07 20:38 +1000
    Re: Help with pipes, buffering and pseudoterminals Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-04-08 12:23 +1200

#88573 — Re: Help with pipes, buffering and pseudoterminals

On Tue, Apr 7, 2015 at 3:48 PM, Cameron Simpson <cs@zip.com.au> wrote:
> The operating system arranges the commection of the shell to the terminal.
> Your usual program has by default a stdin, stdout and stderr. These are
> _all_ the same file handle, duplicated to each of the three file descriptors
> 0, 1 and 2 respectively. On the operating system side, the OS has performed
> _one_ open() call on the terminal device and handed the caller a single file
> descriptor. The caller then calls dup() (or modernly, dup2()) to present the
> open terminal as stdin, stdout and stderr.

Really? I can believe that stdout and stderr are initially duplicates,
but stdin as well? Isn't stdin opened for reading only, and
stdout/stderr for writing only?

I grew up on DOS and OS/2, not on Unix, so maybe there's a massive
simplification here that I'm not aware of. That'd be pretty clean and
tidy if what you say is the case!

ChrisA

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#88636

Chris Angelico wrote:
> Really? I can believe that stdout and stderr are initially duplicates,
> but stdin as well? Isn't stdin opened for reading only, and
> stdout/stderr for writing only?

It depends entirely on how the process that forked your
shell process set things up, but a quick experiment I
just did in a MacOSX terminal window suggests that it's
not really bothered about that distinction:

sh-3.2$ echo foo >&0
foo

-- 
Greg

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