From: daz <dooby@...10.karoo.co.uk>
Date: 2005-07-29T12:56:01+09:00
Subject: Re: redirecting stdout in an extension


Mark Probert wrote:

>
> Hi ..
>
> On Wed, 2005-07-27 at 08:32 +0900, Guillaume Marcais wrote:
> > On Wed, 2005-07-27 at 08:15 +0900, Mark Probert wrote:
> > >
> > > Any ideas on how to redirect stdout in an extension and then revert it
> > > back to its default behaviour?  freopen() redirects, though I am not
> > > sure of returning to normal programming.
> > >
> >
> > Just a guess. I don't know how well it will play with the Ruby
> > interpreter.
> >
> Not so well, unfortunately:
>
>   $ ruby foo.rb
>   test.rb:29:in `write': Bad file descriptor (Errno::EBADF)
>         from test.rb:29:in `puts'
>         from test.rb:29
>
> Thanks for the suggestion.  I would think that there might be some
> facility hidden with the API that might allow this redirection to
> happen, but I have no idea of what it might be ..
>

Hi Mark,

I was thinking that a reply would pop up suggesting rb_io_reopen()
but just discovered what you already know ... it's not exposed :(

Here's my adaptation of a fairly well established method
blagged (including most comments) from the ether.

In short, the chant goes:
  open, dup, dup2, close  <"redirecting">  dup2, close

Will it work in a Ruby ext ?

#-------------------
 require 'redir1'

 puts Time.now
 Time.redir1('foo/blah.txt')
 puts File.read('dummy.tmp')
#-------------------
#-> Fri Jul 29 04:14:10  2005
#-> Fri Jul 29 04:14:10  2005 - foo/blah.txt

Result as expected.  (C below)

daz


<redir1.c>           (almost useless extension)
///////////////////////////////////////////////////////////////////////
#include "ruby.h"
#include <fcntl.h>

int foo_dump(char *fname)
{
    VALUE t = rb_class_new_instance(0, NULL, rb_cTime);

    // time and filename to STDOUT (possibly redirected)
    printf("%s - %s\n", RSTRING(rb_inspect(t))->ptr, fname);

    return 0;
}

VALUE redir1(VALUE self, VALUE o_name)
{
    VALUE rval;       /* our result */
    char *fname;      /* holds the filename */
    int new_fh, sv_so, so_fh = _fileno(stdout);
                 /* OR so_fh = STDOUT_FILENO */

    fname = StringValuePtr(o_name);

    new_fh = open("dummy.tmp", O_CREAT | O_WRONLY, S_IREAD | S_IWRITE);
    if (new_fh < 0)
        rb_sys_fail("can't open file for STDOUT redirection");

    /* create a duplicate handle for standard output */
    sv_so = dup(so_fh);
    /*
       redirect standard output to output file
       by duplicating the file handle onto
       the file handle for standard output.
    */
    dup2(new_fh, so_fh);
    /* close the handle for output file */
    close(new_fh);

    /* will be redirected into output file */
    rval = (foo_dump(fname) == -1) ? Qnil : Qtrue;

    /* restore original standard output handle */
    dup2(sv_so, so_fh);
    /* close duplicate handle for STDOUT */
    close(sv_so);

    return rval;
}

void Init_redir1() {

    rb_define_singleton_method(rb_cTime, "redir1", redir1, 1);
}
///////////////////////////////////////////////////////////////////////