AW: 1.5.25-7 piping directed output to /dev/stdout will not work
Mon Feb 18 13:02:00 GMT 2008
On Feb 16 06:18, Eric Blake wrote:
> The symlink was correctly resolved, and discovered the pipe, but fell back
> to fhandler_base::dup to do the duplication. At any rate, the open
> succeeded, but fd 3 is now tied to a symbolic name
> (/proc/3448/fd/pipe: stands for handle 0x5e0). Then, in the dup2,
> handle 0x5e0 from fd 1 is closed before overlaying fd 3 onto fd 1.
> Finally, the write tries to access the pipe via fd 1, and sends a wakeup
> to the original handle 0x5e0, based on the symbolic name associated with
> fd 3 (now fd 1) - oops; that was the handle that dup already closed.
This might be a bug in the pipe handling, I'm not sure, but the name
associated with the file descriptor is certainly not the reason to access
the 5e0 handle.
> Here, the strace inherits an open handle to f, but does not know which
> file it came from, so it gives up with ENOENT rather than successfully
> creating fd 3. But since strace is obviously able to dup open handles to
> unknown files, it would seem that open(/proc/self/fd/1) to an unknown file
> handle should work as well.
strace doesn't call dup. Rather it just inherits the stdio file handles
to the child through the call to CreateProcess. What you get is a fd 1
in foo which is connected to an unknown file. Calling open("/dev/stdout")
results in a symlink expansion which points into nirvana. The open call
fails before it even tries to call an fhandler's open method.
> Not /dev/stdout, per se, but /proc/<pid>/fd/n. But since /proc is already
> a special device (via fhandler_proc), it seems like such special handling
> should already be happening. Maybe it's just a matter of writing
> fhandler_proc::dup, or making fhandler_proc::open go through the dup
open ("/dev/stdout") goes through the entire symlink expansion first.
fhandler_process is not involved in the open() call, except in the
symlink expansion stage. The final path is the path to the pipe.
Consequentially the open is done by fhandler_pipe::open. It already
knows when it's opening a pipe of the current process and it calls dup().
dup2 is implemented in the outer layer, dtable::dup2. It has no
knowledge about the underlying types of file handlers and usually
doesn't need any. If the target descriptor is opened, it closes it
after duplicating the source handle worked. I don't see that any
special handling at this point would make any sense.
Corinna Vinschen Please, send mails regarding Cygwin to
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