Re: opening standard streams - who does that?

[Marius Nita <marius@cs.pdx.edu>]

On Mon, Jul 08, 2002 at 01:53:29PM -0400, Chuck Winters wrote:
> On the same note, I have a program which uses another library.  Is it possible to
> redirect stderr to a file, and then later set it back to stderr.  I know I can close
> stderr, then reopen stderr to the file I want.  The problem is that when I run the 
> following code, I get a, "No Such File or Directory" error.
> 
> #include <stdlib.h>
> #include <stdio.h>
> 
> int main(void)
> {
> 	FILE *f = NULL;
> 
> 	fclose(stdout);
> 
> 	if( !(f = fopen("/dev/stdout", "w+")) ) {

This gives you a file not found because /dev/stdout is a symlink. Track down
the pts that it points to and open that. On my system it's /dev/pts/12

> 		perror("Opening stdout");
> 		exit(1);
> 	}
> 
> 	fprintf(stdout, "Hey\n");

This won't work because the file pointer "stdout" points to a closed stream.
(You closed it with "fclose(stdout);" above. Remember that "stdout" is just a
FILE * pointer defined in stdio.h.

Above, you opened /dev/pts/XX and pointed f to the new stream. If you say:

        fprintf(f, "Hey\n");

it will work. (Provided that you open the right device file)

> 	exit(0);
> }
> 
> Chuck Winters
> 
> On Sun, Jul 07, 2002 at 08:26:53AM +0100, Glynn Clements wrote:
> > 
> > Suriya Narayanan M S wrote:
> > 
> > > 	I want to know who opens the streams
> > > stdin, stdout, stderr in file-descriptors 1, 2, 3
> > 
> > 0, 1, 2
> > 
> > > and calls the process. I know (right or wrong?)
> > > that on a fork the filedescriptors are duplicated.
> > 
> > Yes.
> > 
> > > So bash or the shell can just forks a new process
> > > without assinging the fds. Is it the kernel
> > > which assigns 0 to the keyboard. How is this
> > > done?
> > 
> > For terminal logins, it's done by getty[1], which is run from the
> > inittab.
> > 
> > [1] "getty" is the generic term; the program may actually be called
> > something else, e.g. agetty, mgetty, uugetty, mingetty.
> > 
> > For telnet/ssh/rsh etc logins, it's done by the daemon (in.telnetd,
> > sshd, in.rshd etc).
> > 
> > Under X, terminal emulators (xterm etc) set up the descriptors. Note
> > that programs which aren't run from a terminal, but from the X session
> > scripts (~/.xsession, ~/.xinitrc) or from the window manager, may not
> > actually have stdin/stdout/stderr assigned.
> > 
> > -- 
> > Glynn Clements <glynn.clements@virgin.net>
> > -
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