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To: cygwin AT cygwin DOT com
Subject: /dev/null regression in Cygwin 3.6.1
Date: Sat, 12 Apr 2025 15:21:12 +0200
Message-ID: <10514352.ICPdZLu4VQ@nimes>
Organization: GNU
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From: Bruno Haible via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Bruno Haible <bruno AT clisp DOT org>
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Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

Hi,

In Gnulib, we have a unit test that compiles the program below as a
native Windows program (either with mingw or with MSVC), that exercises
the Gnulib select() function
  <https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/select.c;h=5a58d6ee47f5b10413d97cb7a1996d424eefc796;hb=HEAD>

Then it does (in a Cygwin shell):

  rm -f t-select-in.tmp
  ./test-select-fd.exe r 0 t-select-in.tmp < /dev/null
  cat t-select-in.tmp

Up to Cygwin 3.6.0, the contents of the file t-select-in.tmp was "1",
like on most other OSes, indicating that the program can read() from
file descriptor 0.

In Cygwin 3.6.1, the contents of the file t-select-in.tmp is "0",
indicating that read()ing from file descriptor 0 would block.
This is a regression.

Looking through the commits between 3.6.0 and 3.6.1, the most likely culprit
is commit d750786e7de013b58e2968eeb6a7fd59dcc535c9 .

Bruno


=============================== test-select-fd.c ===============================
#include <config.h>

#include <stdio.h>
#include <stdlib.h>
#include <sys/select.h>

int
main (int argc, char *argv[])
{
  if (argc == 4)
    {
      char mode = argv[1][0];

      if (mode == 'r' || mode == 'w')
        {
          int fd = atoi (argv[2]);

          if (fd >= 0)
            {
              const char *result_file_name = argv[3];
              FILE *result_file = fopen (result_file_name, "wb");

              if (result_file != NULL)
                {
                  fd_set fds;
                  struct timeval timeout;
                  int ret;

                  FD_ZERO (&fds);
                  FD_SET (fd, &fds);
                  timeout.tv_sec = 0;
                  timeout.tv_usec = 10000;
                  ret = (mode == 'r'
                         ? select (fd + 1, &fds, NULL, NULL, &timeout)
                         : select (fd + 1, NULL, &fds, NULL, &timeout));
                  if (ret < 0)
                    {
                      perror ("select failed");
                      exit (1);
                    }
                  if ((ret == 0) != ! FD_ISSET (fd, &fds))
                    {
                      fprintf (stderr, "incorrect return value\n");
                      exit (1);
                    }
                  fprintf (result_file, "%d\n", ret);
                  exit (0);
                }
            }
        }
    }
  fprintf (stderr, "Usage: test-select-fd mode fd result-file-name\n");
  exit (1);
}
================================================================================




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