/dev/null regression in Cygwin 3.6.1

Sat Apr 12 13:21:12 GMT 2025

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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