how many chars/bytes does STDOUT buffer

sandy105 has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: how many chars/bytes does STDOUT buffer (test) by tye (Sage) on Apr 02, 2015 at 13:35 UTC
`$ perl -MTime::HiRes=sleep -e'while(1){print ".";sleep 0.001}' \| perl + -le'print sysread(STDIN,$rec,1024*1024)' 4096` [download] (no, the sleep isn't strictly necessary, but it makes the buffering effect more obvious.) - tye	[reply] [d/l]
Re^2: how many chars/bytes does STDOUT buffer (test) by choroba (Cardinal) on Apr 02, 2015 at 13:38 UTC
I'm getting 8192. Perl 5.14.4 on Cygwin 64. لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply]
Re^3: how many chars/bytes does STDOUT buffer (test) by BrowserUk (Patriarch) on Apr 02, 2015 at 14:13 UTC
Perl's IO buffer size was changed from 4096 pre-5.14 to 8192 post 5.14. It's also configurable at build time. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked	[reply]
Re^3: how many chars/bytes does STDOUT buffer (docs) by tye (Sage) on Apr 02, 2015 at 13:48 UTC
Yeah, the point was the test, not the sample number produced. Since "line-buffer STDOUT to a terminal but full-buffered to a file or pipe" is the pattern for almost all programs, not just Perl, it is likely that the buffering is not done by Perl and so the size depends on your OS and/or C library version. Though, Perl has several choices of I/O layer implementation and one or more of them might re-implement that standard I/O buffer behavior but might choose different default sizes (though, I recall there being ways to specify buffering and the standard http://perl.plover.com/FAQs/Buffering.html documents such and much more). - tye	[reply]
Re^4: how many chars/bytes does STDOUT buffer (docs) by sandy105 (Scribe) on Apr 03, 2015 at 06:34 UTC
Re^5: how many chars/bytes does STDOUT buffer (docs) by Discipulus (Canon) on Apr 03, 2015 at 09:51 UTC
Some notes below your chosen depth have not been shown here
Re: how many chars/bytes does STDOUT buffer by MidLifeXis (Monsignor) on Apr 02, 2015 at 13:40 UTC
IIRC, it is system dependent, but is usually some multiple of an IO Block Size. --MidLifeXis	[reply]
Re: how many chars/bytes does STDOUT buffer by sandy105 (Scribe) on Apr 03, 2015 at 06:29 UTC
thanks for pointing out that its system dependant .	[reply]
Re^2: how many chars/bytes does STDOUT buffer by ikegami (Patriarch) on Apr 05, 2015 at 06:14 UTC
It's not. See BrowerUk's answer.	[reply]
Re^2: how many chars/bytes does STDOUT buffer by Anonymous Monk on Apr 06, 2015 at 12:29 UTC
Yes, Perl does have its own buffer, in addition to whatever buffering the operating system may or may not do. The safest thing to do is to assume that the size of the buffer is unpredictable. If you need to explicitly flush it, do so, but do not build-in any assumptions as to how large it is.	[reply]
Re^3: how many chars/bytes does STDOUT buffer by ikegami (Patriarch) on Apr 06, 2015 at 19:44 UTC
in addition to whatever buffering the operating system may or may not do. The OS may delay writing it to disk a little after receiving it, but it will appear to be on the disk to anyone trying to read the file immediately after Perl sends it to the OS. That means that if you you use `flush` or `autoflush` to send the data to the OS, it will be immediately available to everyone, though the data could be lost if a power outage occurs shortly after.	[reply] [d/l] [select]