Date: Sat, 4 Mar 2017 20:12:59 -0800 From: Alfred Perlstein <alfred@freebsd.org> To: Julian Elischer <julian@elischer.org>, freebsd-current <freebsd-current@freebsd.org>, Devin Teske <dteske@FreeBSD.org> Subject: Re: Fwd: Re: I/O semantics of pipe and FIFO. Message-ID: <17381773-019a-2181-f00f-1908d04b8d22@freebsd.org> In-Reply-To: <8efdc961-1768-0bc0-715f-4a1e103359d4@elischer.org> References: <20170304214812.GA16845@chaz.gmail.com> <8efdc961-1768-0bc0-715f-4a1e103359d4@elischer.org>
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Devin and I found this when we worked together. I think it was due to some situation in dd(1) where short reads would exit pre-maturely, however I may be mis-remembering. Devin, do you recall the specifics? On 3/4/17 7:44 PM, Julian Elischer wrote: > > an interesting point to discuss? is our behaviour in this test right? > from: "austin-group mailng list (posix standard discussion)" > > ------ rest of email is quoted ------- > On 5/3/17 5:48 am, Stephane Chazelas wrote: > > 2017-03-04 13:14:08 +0000, Danny Niu: >> Hi all. >> >> I couldn't remember where I saw it saying, that when reading >> from a pipe or a FIFO, the read syscall returns the content of >> at most one write call. It's a bit similar to the >> message-nondiscard semantics of dear old STREAM. >> >> Currently, I'm reading through the text to find out a bit >> more, and I appreciate a bit of pointer on this. > [...] > > (echo x; echo y) | (sleep 1; dd count=1 2> /dev/null) > > outputs both x and y in all of Linux, FreeBSD and Solaris in my > tests. > > That a read wouldn't read what's currently in the pipe would be > quite surprising. > > I also wouldn't expect pipes to store the writes as individual > separate message but use one buffer. > > In: > > ( > dd bs=40000 count=1 if=/dev/zero 2> /dev/null > echo first through >&2 > dd bs=40000 count=1 if=/dev/zero 2> /dev/null > echo second through >&2 > ) | (sleep 1; dd bs=100000 count=1 2> /dev/null) | wc -c > > That is where the second write blocks because the pipe is full, > the reading dd still reads both writes in Linux and Solaris in > my tests (on Solaris (10 on amd64 at least), reduce to 20000 > instead of 40000 or both writes would block). > > On FreeBSD, I get only the first write (using 8000 followed by > 10000 for instance). > > FreeBSD is also the only one of the three where > > dd bs=1000000 count=1 if=/dev/zero | dd bs=1000000 count=1 | wc -c > > Doesn't output 1000000. The others schedule both processes back > and forth during their write() and read() system call while the > pipe is being filled and emptied several times. >
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