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comp.lang.ruby

Determining if a String is marshalled

Berger, Daniel

7/15/2003 3:46:00 PM

Hi all,

I was just wondering if there's a way to determine if a string has been
marshalled without visual inspection. Something along the lines of
"string.marshalled?" or "string.kind_of?(MarshaledString)". Is there a
regex I could use?

This had me thinking that it would be nice if Marshal.dump returned a
MarshaledString rather than just a String.

The scenario I'm hitting is a client/server setup where the server may
return a marshaled string or a plain string, depending on the plugin it
calls. At the moment I'm resorting to this, which has potential problems:

r = client.gets
begin
rv = Marshal.load(r)
rescue TypeError
p r
rescue ArgumentError => e
p "R: #{r}"
else
puts rv
end

What do you think?

Regards,

Dan
4 Answers

matz

7/15/2003 3:52:00 PM

0

Hi,

In message "Determining if a String is marshalled"
on 03/07/16, Daniel Berger <djberge@qwest.com> writes:

|I was just wondering if there''s a way to determine if a string has been
|marshalled without visual inspection. Something along the lines of
|"string.marshalled?" or "string.kind_of?(MarshaledString)". Is there a
|regex I could use?

Marshal string have its format version embedded at the top 2 bytes (no
magic though). So that

string[0,2] == Marshal.dump("")[0,2]

should work for most of the cases.

matz.

Brian Candler

7/15/2003 3:54:00 PM

0

On Wed, Jul 16, 2003 at 12:45:46AM +0900, Daniel Berger wrote:
> I was just wondering if there''s a way to determine if a string has been
> marshalled without visual inspection. Something along the lines of
> "string.marshalled?" or "string.kind_of?(MarshaledString)". Is there a
> regex I could use?

Check for the first two bytes, which are the major/minor version number.
Rather than hard-code it, you could do

marshall_sig = Marshal.dump("")[0..1]
marshall_check = /\A#{marshall_sig}/

irb(main):004:0> "abc" =~ marshall_check
=> nil
irb(main):005:0> Marshal.dump(123) =~ marshall_check
=> 0

Of course, the string might actually start with those bytes, but they are
low ("\004\006" on my machine) so unlikely to occur in a normal string.

Regards,

Brian.

Berger, Daniel

7/15/2003 4:38:00 PM

0



Brian Candler wrote:
> On Wed, Jul 16, 2003 at 12:45:46AM +0900, Daniel Berger wrote:
>
>>I was just wondering if there''s a way to determine if a string has been
>>marshalled without visual inspection. Something along the lines of
>>"string.marshalled?" or "string.kind_of?(MarshaledString)". Is there a
>>regex I could use?
>
>
> Check for the first two bytes, which are the major/minor version number.
> Rather than hard-code it, you could do
>
> marshall_sig = Marshal.dump("")[0..1]
> marshall_check = /\A#{marshall_sig}/
>
> irb(main):004:0> "abc" =~ marshall_check
> => nil
> irb(main):005:0> Marshal.dump(123) =~ marshall_check
> => 0
>
> Of course, the string might actually start with those bytes, but they are
> low ("\004\006" on my machine) so unlikely to occur in a normal string.
>
> Regards,
>
> Brian.

Thanks Brian and Matz for the info. My only concern might be that a
pack''d string would inadvertantly pass this test. I guess that''s what
begin/rescue is for, though. :)

Regards,

Dan

Hal E. Fulton

7/15/2003 5:38:00 PM

0

----- Original Message -----
From: "Daniel Berger" <djberge@qwest.com>
To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
Sent: Tuesday, July 15, 2003 11:38 AM
Subject: Re: Determining if a String is marshalled


> > Of course, the string might actually start with those bytes, but they
are
> > low ("\004\006" on my machine) so unlikely to occur in a normal string.
> >
> > Regards,
> >
> > Brian.
>
> Thanks Brian and Matz for the info. My only concern might be that a
> pack''d string would inadvertantly pass this test. I guess that''s what
> begin/rescue is for, though. :)

Hmm, assuming a packed string can have an arbitrary value,
this could easily happen (probability 1/65536).

Here''s a silly idea. When you Marshal a
string, add a singleton to it (that does
nothing). Test for that later.

str.respond_to? :marshalled # it''s a marshalled string

However, that only works for strings in
memory, of course. If you read it in from
a file... who knows?

Hal

--
Hal Fulton
hal9000@hypermetrics.com