[TUHS] FD 2

[Warner Losh]

On Sat, Jan 21, 2023, 10:34 AM Warner Losh <imp at bsdimp.com> wrote:

>
>
> On Sat, Jan 21, 2023, 8:45 AM Clem Cole <clemc at ccc.com> wrote:
>
>> Certainly fd 2 as a error output appears with Lesk's portable C library,
>> which was included in V6 - see the last para on page 1 of his document:
>>
>> "Initially you are given three file descriptors by the system: 0, 1, and
>> 2. File 0 is the standard input; it is normally the teletype in
>> time-sharing or input data cards in batch. File 1 is the standard output;
>> it is nor- mally the teletype in time-sharing or the line printer in batch.
>> File 2 is the error file; it is an output file, nor- mally the same as file
>> 1, except that when file 1 is diverted via a command line ’>’ operator,
>> file 2 remains attached to the original destination, usually the terminal.
>> It is used for error message output. These popular UNIX conventions are
>> considered part of the C library specification. By closing 0 or 1, the
>> default input or output may be re-directed; this can also be done on the
>> command line by >file for output or <file for input."
>>
>> This document pre-dates ditroff.   The idea/names of stdin/out/err (as
>> opposed to fd 0, 1, 2) does not come about until Dennis does "typesetter C"
>> which is described in K&R.
>>
>> That said, my memory is fd 2 as ana error path was there in Fourth or
>> Fifth Edition, before Lesk did this library -- i.e. it was a convention
>> that Dennis/Ken et al were all using -- when it is realized that printing
>> errors on in the same output flow as the standard (normal) output was
>> problematic in a pipe stream.
>> ᐧ
>>
>
> It looks to be a mixed bag in 5th edition. Cc, ac, comm and find all
> printf the errors. Diff uses writes to fd 2. I didn't catalog further, but
> this kinda looks like a developing convention, not yet fully realized. .
>

Further digging shows v6 is quite similar. V7 revamps everything, with
printf changed to a routine that writes to stderr. Each program did this
differently.

Granted, this is a small sample size. There wasn't a huge uptake of
stderr/fd2 in v6 it seems, but v7 looks like it had a pass over the code to
at least try for all errors going to fd2.

Warner


> Warner
>
> On Fri, Jan 20, 2023 at 5:46 PM ron minnich <rminnich at gmail.com> wrote:
>>
>>> I am trying to remember when fd 2 (aka stderr) became a thing. I have a
>>> vague memory that it was post-v6 but that may be way off.
>>>
>>>
>>>
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