Why stdout_write checks for terminal?

Why stdout_write checks for terminal?

[Sergey Dmitrouk]

Hi,

the code in src/stdio/__stdout_write.c checks whether output is going to
a terminal and if it's not the case disables line buffering.  I'm
wondering what's the reason behind this?  This causes some programs to
produce different output depending whether stdout is terminal or not,
not a bit deal, but I don't see much profit in disabling buffering
either.

Regards,
Sergey

Re: Why stdout_write checks for terminal?

[Josiah Worcester]

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On Tue, Oct 21, 2014 at 9:56 AM, Sergey Dmitrouk <sdmitrouk@accesssoftek.com
> wrote:

> Hi,
>
> the code in src/stdio/__stdout_write.c checks whether output is going to
> a terminal and if it's not the case disables line buffering.  I'm
> wondering what's the reason behind this?  This causes some programs to
> produce different output depending whether stdout is terminal or not,
> not a bit deal, but I don't see much profit in disabling buffering
> either.
>
> Regards,
> Sergey
>

From the spec <
http://pubs.opengroup.org/onlinepubs/009695399/functions/stdin.html>:  "the
standard input and standard output streams are fully buffered if and only
if the stream can be determined not to refer to an interactive device."

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Re: Why stdout_write checks for terminal?

[Rich Felker]

On Tue, Oct 21, 2014 at 05:56:40PM +0300, Sergey Dmitrouk wrote:
> Hi,
> 
> the code in src/stdio/__stdout_write.c checks whether output is going to
> a terminal and if it's not the case disables line buffering.  I'm
> wondering what's the reason behind this?  This causes some programs to
> produce different output depending whether stdout is terminal or not,
> not a bit deal, but I don't see much profit in disabling buffering
> either.

Full buffering _must_ be disabled if the underlying file is an
"interactive device" (terminal); this is a requirement of the
standards. Somewhere in the distant past musl always put stdout in
line-buffered mode, but users complained (rightfully) because programs
writing binary data to stdout (e.g. things like djpeg) were 10-100x
slower than with other libcs, so now whether it's line or full
buffered depends on whether it's a terminal.

Rich

Re: Why stdout_write checks for terminal?

[Sergey Dmitrouk]

On Tue, Oct 21, 2014 at 08:17:27AM -0700, Rich Felker wrote:
> Full buffering _must_ be disabled if the underlying file is an
> "interactive device" (terminal); this is a requirement of the
> standards. Somewhere in the distant past musl always put stdout in
> line-buffered mode, but users complained (rightfully) because programs
> writing binary data to stdout (e.g. things like djpeg) were 10-100x
> slower than with other libcs, so now whether it's line or full
> buffered depends on whether it's a terminal.

Thanks for the explanation (and to Josiah as well), that makes sense.
Related commit message is missing details and it wasn't really clear
why this change is needed.

Regards,
Sergey