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List:       linux-man
Subject:    AW: sendfile() more detailed information ? anyone ?
From:       Walter Harms <wharms () bfs ! de>
Date:       2021-01-21 8:51:07
Message-ID: f1652c86c31a42378d2ca5e07a76d2f2 () bfs ! de
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I would suggest to search the busybox ml for the thread:
 Why doesn't "cat a b > foo" work?

I did some search on the net for sendmail() and found that
several project had problems with various implementations
of sendfile() and offered a switch to disable its use.
(I added the apache doc for more details but other project
 have the same). Nothing is mentioned in man page but to be fair
several problems have been fixed.

re,
 wh

________________________________________
Von: Michael Kerrisk (man-pages) <mtk.manpages@gmail.com>
Gesendet: Mittwoch, 20. Januar 2021 21:18
An: Walter Harms; linux-man@vger.kernel.org
Cc: mtk.manpages@gmail.com
Betreff: Re: sendfile() more detailed information ? anyone ?

Hello Walter,

On 1/20/21 6:40 PM, Walter Harms wrote:
> Hi List,
> there was a discussion on the busybox-ml about a kernel-bug in sendfile()=
.
> <cite>
> In 2.6.33.7, when used with a normal file as the output, it always
> writes the data to offset 0 in the file.
>
> That's fine if you only call it once and then close the output
> fd. However, if you call it multiple times (as busybox 'cat' does when
> copying multiple files), then you don't get the expected result. The
> contents of the input files are not concatenated. Instead the contents
> of each input file overwites the existing contents of the output file
> starting at offset 0.
> </cite>

At a very quick test, I could not reproduce this. Do you have a URL
for the mail discussion?

Thanks,

Michael

> these apache doc mentions more problems
> https://httpd.apache.org/docs/2.4/mod/core.html#enablesendfile
>
> It seems that the bug mentioned above is fixed but nobody seems to
> know when this was done.
>> IMHO this should be mentioned in the man page, but i lack detailed
> information and time to research them. I would like to encourage
> anyone who has information to  come forward so it can be added to man
> page.>
> re,
>  wh
>


--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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