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Date: Mon, 14 Apr 2025 18:12:35 +0900
To: cygwin AT cygwin DOT com
Subject: Re: /dev/null regression in Cygwin 3.6.1
Message-Id: <20250414181235.ec84c66f2bec856c0b4a9f44@nifty.ne.jp>
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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Hi Bruno,

On Sat, 12 Apr 2025 15:21:12 +0200
Bruno Haible wrote:
> 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);
> }
> ================================================================================

Thanks for the report.
I tryied to compile your testcase as mingw binary, but
config.h and sys/select.h are missing from mingw.

Could you plsese let us know how to compile it?

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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