Line-by-line processing when stdin is not a tty

Nobody nobody at nowhere.com
Thu Aug 12 02:42:51 CEST 2010


On Wed, 11 Aug 2010 10:32:41 +0000, Tim Harig wrote:

>>> Usually you either
>>> need an option on the upstream program to tell it to line
>>> buffer explicitly
>>
>> once cat had an option -u doing exactly that but nowadays
>> -u seems to be ignored
>>
>> http://www.opengroup.org/onlinepubs/009695399/utilities/cat.html
> 
> I have to wonder why cat knows or cares.

The issue relates to the standard C library. By convention[1], stdin and
stdout are line-buffered if the descriptor refers to a tty, and are
block-buffered otherwise. stderr is always unbuffered.

Any program which uses stdin and stdout without explicitly setting the
buffering or using fflush() will exhibit this behaviour.

[1] ANSI/ISO C is less specific; C99, 7.19.3p7:

	As initially opened, the standard error stream is not fully
	buffered; 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.

POSIX says essentially the same thing:

<http://www.opengroup.org/onlinepubs/9699919799/functions/stdin.html>




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