Is STDERR buffered?

vsespb has asked for the wisdom of the Perl Monks concerning the following question:

This documentation says it's not:

http://www.plover.com/FAQs/Buffering.html

There's one other exception to the rule that says that filehandles are cold unless they're attached to the terminal: The filehandle STDERR, which is normally used for error logging, is always in line buffered mode by default.

But there are example code in perldoc, which make me think it's sometimes buffered

http://perldoc.perl.org/functions/select.html

use IO::Handle; STDERR->autoflush(1);

Also this blogpost claims that it does

http://blog.famzah.net/2010/04/08/auto-flush-both-stdout-and-stderr-in-perl/

In my tests it's always line buffered, even when not attached to terminal, but I wonder if this is always true or not.

Also I have an error in one of my scripts, which might be related to this (process terminated with signal and STDERR output truncated, even if contains newlines), but I can't reproduce it so can't investigate

Comment on Is STDERR buffered?

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Re: Is STDERR buffered? by Anonymous Monk on May 24, 2013 at 14:56 UTC
But there are example code in perldoc, which make me think it's sometimes buffered line-buffered means it buffered, so the article says its line-buffered, the code in select shows how to unbuffer it -- do you get it now? In my tests it's always line buffered, even when not attached to terminal, but I wonder if this is always true or not. Here is a start in finding out, it depends on your platform UTSL :) http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=stderr.%2Bbuf&sr=1 http://perl5.git.perl.org/perl.git/blob?f=Porting/Glossary#l1937 `1937 d_setlinebuf (d_setlnbuf.U): 1938 This variable conditionally defines the HAS_SETLINEBUF sy +mbol, which 1939 indicates to the C program that the setlinebuf() routine +is available 1940 to change stderr or stdout from block-buffered or unbuffe +red to a 1941 line-buffered mode. 1942` [download] C file input/output	[reply] [d/l]
Re^2: Is STDERR buffered? by vsespb (Chaplain) on May 25, 2013 at 13:25 UTC
Ok, Thanks! Indeed, it can be line-buffered, block-buffered or unbuffered. So, perldoc example make sense. But blogpost above talks about block-buffering, I believe. Anyway I'll try check perl source. Thanks!	[reply]