On Tue, 12 Jun 2001, Avram Aelony <[EMAIL PROTECTED]> wrote,
> I agree with you, but my question really is this: What happens if there is no
> open() statement at all??
> What if you *only* have this: print FILE "test\n";
>
> Does the string "test" get executed and go nowhere.... or .... does the print
> statement get passed over when perl sees a filehandle FILE that has never been
> defined???
I number the examples below. Please look at number 2 and 3. There's no
open() statement there. And see what perl thinks about ACT_AS_FILEHANDLE.
Perl refuses to print to undefined filehandle, or, undefined file descriptor.
Or, more presicely: it only prints to opened filehandle and only opened
for output.
You can check the file descriptor using fileno() on STDOUT (1), opened
filehandle (usually > 2), and ACT_AS_FILEHANDLE (undef).
hth
s::a::n
--
http://www.trabas.com
> Hasanuddin Tamir wrote:
>
> > On Tue, 12 Jun 2001, Avram Aelony <[EMAIL PROTECTED]> wrote,
> >
> > > Hi,
> > >
> > > I commonly make use of statements like: print OUTFILE "Print
> > > results, data, or anything else\n";
> > > This is normally preceded by defining my file handle, as in something
> > > like: open(OUTFILE, ">newfile.txt");
> > >
> > > I'd like to know what *actually* happens internally if the filehandle,
> > > OUTFILE in this case, is not defined ahead of time?
> > > The output is shut off, but I would like to know if this is a good way
> > > to control output.
> >
> > Ok, let's examine some examples.
> >
1.
> > #!/usr/bin/perl
> > # no -w, no warning
> > open FILE, ">/etc/passwd"; # no error checking
> > # should be permission denied for
> > # ordinary user, and disaster for
> > # for superuser
> > print FILE "sometext\n";
> >
> > The output goes nothing.
> >
2.
> > perl -e 'print ACT_AS_FILEHANDLE "sometext\n";
> >
> > No -w, prints nothing.
> >
> > But watch this,
> >
3.
> > $ perl -we 'print ACT_AS_FILEHANDLE "sometext\n";
> >
> > The -w switch tell Perl to warn you about what it thinks goes odd
> > (this is not the output of the code),
> >
> > Name "main::ACT_AS_FILEHANDLE" used only once: possible typo at -e line 1.
> > Filehandle main::ACT_AS_FILEHANDLE never opened at -e line 1
> >
4.
> > $ perl -we 'open FILE, ">/etc/passwd"; print FILE "sometext\n";'
> >
> > The same goes here,
> >
> > print on closed filehandle main::FILE at -e line 1.
> >
> > And when you a bit differently (not the "or die $!"),
> >
5.
> > $ perl -we 'open FILE, ">/etc/passwd" or die $!; print FILE "sometext\n";'
> >
> > The code stops with message,
> >
> > Permission denied at -e line 1.
> >
> > This error message comes from $!. This is a control statement that works
> > like,
> >
> > if (open FILE, ">/my/home/lets/use/another_file") {
> > print FILE "sometext\n";
> > } else {
> > die $!;
> > }
> >
> > So when the open() doesn't work, you won't have the warning (even when
> > you put -w) since the print statement never executes.
> >
> > The morals of the story:
> >
> > * always use -w switch
> > * always check the return value of system call such as open()
> >
> > >From the perldoc -f open,
> >
> > "When opening a file, it's usually a bad idea to continue normal
> > execution if the request failed, so open() is frequently used in
> > connection with die()."
> >
> > That's what you need to control the output.
> >
> > hth
> > s::a::n
> > --
> > http://www.trabas.com
>
