Date: Tue, 15 Jun 2021 00:06:25 -0700 From: "Ronald F. Guilmette" <rfg@tristatelogic.com> To: FreeBSD Questions <freebsd-questions@freebsd.org> Subject: Re: Is a successful call to write(2) atomic? Message-ID: <22440.1623740785@segfault.tristatelogic.com> In-Reply-To: <CAFbbPuh1xdoXqxjoEQHyPXVqX=KayokPgmpupEYovSHtcD4P5A@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
In message <CAFbbPuh1xdoXqxjoEQHyPXVqX=3DKayokPgmpupEYovSHtcD4P5A@mail.gma=
il.com>, =
Paul Procacci <pprocacci@gmail.com>wrote:
>I had a long response, but decided to shorten it....I hope you don't mind=
.
Not at all.
>The only time atomicity is guaranteed is for pipes when the write(2) size
>fits in PIPE_BUF (512 bytes on my version of FreeBSD).
>Otherwise, it depends is really the best answer as it depends on the file
>system.
>Sometimes filesystems like ext4 require O_DIRECT to be set, in order to
>avoid the garbled text as you've mentioned, and even then it's limited to
>the number of bytes written.
This is deeply distrubing. I never knew about this. I am furiously readi=
ng
the FreeBSD & Linux man pages for open(2). (It appears that both support
that flags, O_DIRECT and O_SYNC, aqnd Linux also adds the O_DSYNC flag.
I feel a bit dumb that I never nknew about these until now.)
>A suggestion; the way I've done something similar in the past.....
>
>Parent Process - Orchestrator - creates pipes handing one end of the pipe
>to each child.
>Child Process - do'ers - write whatever it intends on writing to the pipe
>the parent is waiting for data on.
>Parent process then grabs the data the child wrote, and in turn writes it
>to stdout (or whatever file descriptor you intend).
Yes, yes. Believe me when I say that I am considering reworking my code
to do exactly this as we speak. I don't even know for sure if I have a
"{lack of} atomicity" problem in my existing code, but I am 100% sure
that what you just described would completely solve the problem, if indeed
it does exist.
>This method, a bit more involved, ensures one writer to a file descriptor=
,
>is portable, and you can rest easy about it working on whatever latest an=
d
>greatest file system someone decides to make.
Yes. Thank you.
Regards,
rfg
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?22440.1623740785>