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Date: Thu, 07 Apr 2016 23:02:50 +0200
From: "Juan Manuel Guerrero (juan DOT guerrero AT gmx DOT de) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
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To: djgpp AT delorie DOT com
Subject: Different dup2 behaviour between DOS/DJGPP and posix.
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Reply-To: djgpp AT delorie DOT com

Please note the following code snippet:

#include <stdio.h>
#include <errno.h>
#include <unistd.h>
int
main (void)
{
int result = 0;
int fd;
FILE *f = fopen("./dup2.txt", "w");
errno = 0;
              if ((fd = dup2 (1, 1)) != 1)
                result |= 2;
fprintf(f, "%d:  result = %d fd = %d  errno = %s\n", __LINE__, result, fd, sys_errlist[errno]);


              close (0);
errno = 0;
              if ((fd = dup2 (0, 0)) != -1)
                result |= 8;
fprintf(f, "%d:  result = %d fd = %d  errno = %s\n", __LINE__, result, fd, sys_errlist[errno]);

              return result;
}

It depicts an autoconf check for dup2.  If I compile this with DJGPP I get
the following output:
14:  result = 0 fd = 1  errno = No Error
19:  result = 8 fd = 0  errno = No Error

If I do the same on linux I get the following output:
19:  result = 0 fd = 1  errno = Success
24:  result = 0 fd = -1  errno = Bad file descriptor

The question is why DOS/DJGPP allows to close STDIN and reopen it with FD 0
meanwhile posix seems not to allow this.  Is this behaviour expected?
I have inspected the code of dup2 and the implementation seems OK.

The consequence of this check is that Autoconf assumes that DJGPP's dup2
implementaion does not work.


Regards,
Juan M. Guerrero