AW: 1.5.25-7 piping directed output to /dev/stdout will not work

Leutwein Tobias (BEG-PG/EES) Tobias.Leutwein@de.bosch.com
Fri Feb 15 19:02:00 GMT 2008


I did this examples under the bash.
Attached is a cygcheck output of a cygwin 1.5.24 installation. With this version it works.

Tobias

> -----Ursprüngliche Nachricht-----
> Von: Eric Blake [mailto:ebb9@byu.net] 
> Gesendet: Freitag, 15. Februar 2008 15:01
> An: cygwin@cygwin.com; Leutwein Tobias (BEG-PG/EES)
> Betreff: Re: 1.5.25-7 piping directed output to /dev/stdout 
> will not work
> 
> According to Leutwein Tobias (BEG-PG/EES) on 2/14/2008 11:51 PM:
> | Since I updated to 1.5.25 piping directed output to /dev/stdout with
> | bash 3.2.33-18 will not work.
> 
> Updated from what prior version?
> 
> |
> |> echo "test string" > /dev/stdout | wc -w
> | v:\Programme\cygwin\current\bin\bash: echo: write error: Bad file
> | descriptor
> | 0
> 
> Hmm.  The error message does not list a POSIX-style path to 
> bash.  Is this
> started directly under cmd.exe?  At any rate, you've told 
> bash to do the
> following:
> 
> create a pipe, connecting echo's stdout to wc's stdin
> within echo, open /dev/stdout at the next available fd (3), 
> as a clone of fd 1
> call dup2(3,1), which closes 1 then moves 3 in its place
> 
> And it looks like what cygwin is doing is closing the handle 
> owned by fd
> 1, so that the handle owned by fd 3 is invalid before it is 
> reassociated
> with fd 1, explaining the bad fd write error.
> 
> True - on some Unix systems, /dev/stdout is a device rather than a
> symlink, and as such, the OS goes to great pains to keep the 
> original fd 1
> when going through the name /dev/stdout.  And I'm not sure that this
> problem is cygwin specific.  At any rate, it does not violate 
> POSIX - you
> are in the realm of unportable behavior when you rely on /proc (or on
> /dev/stdout).
> 
> On the other hand, this behavior is indeed different than what Linux
> provides.  I tested the same scenario on Linux, and it 
> appears to special
> case dup2 when moving the cloned fd back on top of the original, the
> underlying handle is not invalidated.  Which means that the 
> bug might lie
> in cygwin1.dll and not in bash.
> 
> On the other hand, the following C program behaves correctly when
> simulating the same sequence of actions (although it avoids 
> the unportable
> use of /proc).  I'll have to investigate further to prove 
> where the blame
> lies, but right now, it seems that it is in cygwin, for not treating
> open("/proc/self/fd/1") as equivalent to dup(1).
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/stat.h>
> #include <errno.h>
> #include <string.h>
> #include <unistd.h>
> #include <fcntl.h>
> 
> int
> main (int argc, char* argv[])
> 
> ~  int f = dup (1);
> ~  if (f < 0)
> ~    fprintf (stderr, "failed to dup stdout, %d\n", errno);
> ~  f = dup2 (f, 1);
> ~  if (f < 0)
> ~    fprintf (stderr, "failed to dup2 back to stdout, %d\n", errno);
> ~  f = write (1, "hi\n", 3);
> ~  if (f < 3)
> ~    fprintf (stderr, "failed to write to new stdout, %d\n", errno);
> ~  return 0;
> 
> 
> --
> Don't work too hard, make some time for fun as well!
> 
> Eric Blake             ebb9@byu.net
> 
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