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To: "'Ken Brown'" <kbrown AT cornell DOT edu>
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In-Reply-To: <b311d907-7376-5bc6-3216-7d2b96728dbc@cornell.edu>
Subject: Sv: Sv: Sv: Named pipes and multiple writers
Date: Fri, 27 Mar 2020 15:53:36 +0100
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From: Kristian Ivarsson via Cygwin <cygwin AT cygwin DOT com>
Reply-To: sten DOT kristian DOT ivarsson AT gmail DOT com
Cc: "'cygwin'" <cygwin AT cygwin DOT com>
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>On 3/26/2020 7:19 PM, Ken Brown via Cygwin wrote:
>> On 3/26/2020 6:39 PM, Ken Brown via Cygwin wrote:
>>> On 3/26/2020 6:01 PM, sten DOT kristian DOT ivarsson AT gmail DOT com wrote:
>>>> The ENIXIO occurs when parallel child-processes simultaneously =
using=20
>>>> O_NONBLOCK opening the descriptor.
>>>
>>> This is consistent with my guess that the error is generated by=20
>>> fhandler_fifo::wait.=A0 I have a feeling that read_ready should have =

>>> been created as a manual-reset event, and that more care is needed =
to=20
>>> make sure it's set when it should be.
>>>
>>>> I could provide a code-snippet
>>>> to reproduce it if wanted ?
>>>
>>> Yes, please!
>>=20
>> That might not be necessary.=A0 If you're able to build the git repo=20
>> master branch, please try the attached patch.

>Here's a better patch.


I finally succeeded to build latest master (make is not my favourite =
tool)
and added the patch, but still no success in my little test-program (see
attachment) when creating a write-file-descriptor with O_NONBLOCK

=20
>Ken

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#include <cstring>

#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#include <iostream>
#include <string>

namespace
{
   auto error(const int error)
   {
      std::cerr << getpid() << "\terror:\t" << error << '\t' << =
std::strerror(error) << std::endl;
      return error;
   }
}

int main()
{
   const auto name{"/tmp/my_name"};

   const auto result =3D mkfifo(name, 0666);

   if(result) return error(errno);

   constexpr auto writers{5};
   constexpr auto messages{4};

   for(auto idx =3D 0; idx < writers; ++idx)
   {
      const auto pid =3D fork();

      if(pid < 0) return error(errno);

      if(pid =3D=3D 0)
      {
         std::cout << "child " << getpid() << std::endl;
         for(auto idx =3D 0; idx < messages; ++idx)
         {
            const auto wfd =3D open(name, O_WRONLY | O_NONBLOCK);
            if(wfd < 0) return error(errno);
            const auto msg{std::to_string(getpid())};
            if(write(wfd, msg.data(), msg.size() + 1) < 0) error(errno);
            close(wfd);
         }
         return 0;
      }
   }

   {
      std::cout << "parent" << std::endl;

      const auto rfd =3D open(name, O_RDONLY);
      const auto wfd =3D open(name, O_WRONLY);
      if(rfd < 0) return error(errno);
      for(auto idx =3D 0; idx < writers * messages; ++idx)
      {
         std::string buffer;
         buffer.resize(80);
         if(read(rfd, &buffer[0], buffer.size()) < 0) error(errno);
         std::cout << buffer << std::endl;
      }
      close(wfd);
      close(rfd);
   }

   if(unlink(name) < 0) return error(errno);

   return 0;
}
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