From: Jos Backus <jos@...>
Date: 2004-12-06T15:57:38+09:00
Subject: Re: Obtaining mode information on a File object

On Sun, Dec 05, 2004 at 11:32:36PM +0900, Robert Klemme wrote:
> 
> "Jos Backus" <jos@catnook.com> schrieb im Newsbeitrag 
> news:20041204195115.GA83981@lizzy.catnook.com...
> >Another question... How do I access the File open mode ("w" in this case) 
> >on a
> >File object?  Example:
> >
> >class Foo
> > def initialize(file)
> >   begin
> >     yield self
> >   ensure
> >     # We only want to call finalize when the file was opened for writing;
> >     # The following doesn't work, but how _do_ you do this?
> >     finalize(file) if file.mode.writable? # file was opened with mode "w"
> >   end
> > end
> > def finalize(file)
> >   file.puts "END"
> > end
> >end
> >
> >File.open("foo", "w") do |file|
> > a = Foo.new(file) do
> >   # ... Use a ...
> > end
> >end
> 
> First of all, I know no other way than to just try to write to the file and 
> see what happens.  Maybe writing an empty string is a solution on some 
> (all?) platforms.
 
I hadn't thought of that. It seems... wrong. This information is available
already, it's a matter of making it accessible using an attribute reader.

> But I think usually you don't need that info: if a method requires an IO 
> object open for writing then it simply uses it that way and you get bitten 
> by an exception if it's not open for writing.  Similar applies for files 
> needed for reading.
> 
> What is the scenario where you need that?

I'm writing a class to handle CPIO archives. Iff the file passed in was opened
for writing I want to call a finalizer that appends a trailer, as in the above
code snippet.

I'm currently looking at io.c to see if the vmode variable in rb_open_file
can't be exported somehow, like filename. One issue is that the mode argument
can be either a String or Fixnum (things like O_RDONLY), which complicates
matters.

Thanks,
-- 
Jos Backus                       _/  _/_/_/      Sunnyvale, CA
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jos at catnook.com        _/_/   _/_/_/          require 'std/disclaimer'