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Subject: Re: name pipe problem: 1 writer, multiple concurrent readers
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From: Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Ken Brown <kbrown AT cornell DOT edu>
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On 5/19/2020 7:26 AM, Morten Kjærulff via Cygwin wrote:
> Hi,
> 
> The following script:
> 
> #!/bin/sh
> 
> rm -f t.pip t.txt
> 
> mkfifo t.pip
> printf "line1\nline2\n" >t.txt
> 
> ps
> 
> {
>    while true ; do
>      cp t.txt t.pip
>    done
> } &
> 
> rm -f t.rc.*
> for rc in 0 1 2 3 4 5 6 7 8 9 ; do
>    {
>      diff t.pip t.txt
>      echo $? >t.rc.$rc
>    }
> done
> echo result1 start
> ls  t.rc.* | wc -l
> cat t.rc.*
> echo result1 end
> 
> rm -f t.rc.*
> for rc in 0 1 2 3 4 5 6 7 8 9 ; do
>    {
>      diff t.pip t.txt
>      echo $? >t.rc.$rc
>    } & # run the readers in parallel
> done
> sleep 10
> echo result2 start
> ls  t.rc.* | wc -l
> cat t.rc.*
> echo result2 end
> 
> ps
> 
> Give me output like this:
> 
> $ ./tpip.sh
>        PID    PPID    PGID     WINPID   TTY         UID    STIME COMMAND
>       1642    1600    1642      65264  cons1    1058872 13:18:58 /usr/bin/sh
>       1600       1    1600      72728  cons1    1058872 13:18:33 /usr/bin/bash
>       1645    1642    1642      42088  cons1    1058872 13:18:58 /usr/bin/ps
> result1 start
> 10
> 0
> 0
> 0
> 0
> 0
> 0
> 0
> 0
> 0
> 0
> result1 end
> 0a1,2
>> line1
>> line2
> diff: t.pip
> result2 start
> 1
> 2
> result2 end
>        PID    PPID    PGID     WINPID   TTY         UID    STIME COMMAND
>       1690    1688    1642     104032  cons1    1058872 13:19:01 /usr/bin/diff
>       1681    1642    1642      95012  cons1    1058872 13:19:00 /usr/bin/sh
>       1642    1600    1642      65264  cons1    1058872 13:18:58 /usr/bin/sh
>       1684    1681    1642      99624  cons1    1058872 13:19:00 /usr/bin/diff
>       1678    1676    1642      94532  cons1    1058872 13:19:00 /usr/bin/diff
>       1688    1642    1642      88864  cons1    1058872 13:19:01 /usr/bin/sh
>       1698    1642    1642     104820  cons1    1058872 13:19:11 /usr/bin/ps
>       1692    1642    1642      66572  cons1    1058872 13:19:01 /usr/bin/sh
>       1677    1674    1642      86692  cons1    1058872 13:19:00 /usr/bin/diff
>       1646    1642    1642      30888  cons1    1058872 13:18:58 /usr/bin/sh
>       1600       1    1600      72728  cons1    1058872 13:18:33 /usr/bin/bash
>       1686    1685    1642      14320  cons1    1058872 13:19:01 /usr/bin/diff
>       1685    1642    1642      25608  cons1    1058872 13:19:00 /usr/bin/sh
>       1676    1642    1642     104212  cons1    1058872 13:19:00 /usr/bin/sh
>       1689    1642    1642      98004  cons1    1058872 13:19:01 /usr/bin/sh
>       1674    1642    1642      44152  cons1    1058872 13:19:00 /usr/bin/sh
>       1680    1646    1642      28224  cons1    1058872 13:19:00 /usr/bin/cp
>       1682    1679    1642      43612  cons1    1058872 13:19:00 /usr/bin/diff
>       1694    1692    1642      67736  cons1    1058872 13:19:01 /usr/bin/diff
>       1683    1642    1642      93544  cons1    1058872 13:19:00 /usr/bin/sh
>       1679    1642    1642      90188  cons1    1058872 13:19:00 /usr/bin/sh
>       1691    1689    1642      68560  cons1    1058872 13:19:01 /usr/bin/diff
>       1687    1683    1642      83952  cons1    1058872 13:19:01 /usr/bin/diff
> 
> $
> 
> That is, when I run the readers in sequence, no problem, but when I
> run the in parallel, they either hang or t.pip seems empty.

The current cygwin release doesn't support multiple concurrent readers of a 
FIFO.  I have recently added that support, which should appear in the next 
release, although there are still bugs to be fixed.  See

   https://cygwin.com/pipermail/cygwin-patches/2020q2/010195.html

Ken
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