Using pack/unpack to go from string to address and back to s

Ruby
1

Hi all,

Is there a way, given a pointer address, to get back to the original
object? I’m thinking specifically of strings here.

irb(main):006:0> str = “hello”
=> “hello”
irb(main):007:0> addr = [str].pack(‘p*’).unpack(‘l’)
=> [43397704]

Is there a way to go from that address back to “hello”? You can’t
simply reverse it:

irb(main):025:0> addr.pack(‘l’).unpack(‘p*’)
ArgumentError: no associated pointer

It was pointed out to me that Ruby uses rb_str_associate and
rb_str_associated behind the scenes with regards to pack(‘p’), but I’m
also wondering about addresses for strings that weren’t obtained via
pack(‘p*’).

Or is this simply impossible?

Thanks,

Dan

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2

Hi,

At Wed, 21 Jun 2006 23:22:17 +0900,
Berger, Daniel wrote in [ruby-talk:198375]:

It was pointed out to me that Ruby uses rb_str_associate and
rb_str_associated behind the scenes with regards to pack(‘p’), but I’m
also wondering about addresses for strings that weren’t obtained via
pack(‘p*’).

Or is this simply impossible?

It’s disallowed. If it were possible, you could get an illegal
“String.” For instance:

[1].pack(‘l’).unpack(‘p*’)

3

[email protected] wrote:

Or is this simply impossible?

It’s disallowed. If it were possible, you could get an illegal
“String.” For instance:

[1].pack(‘l’).unpack(‘p*’)

Would it be possible to make an exception for strings (char pointers)?
Or is there no way to determine the type of underlying object in time?

Dan

4

Hi,

At Fri, 23 Jun 2006 02:06:26 +0900,
Daniel B. wrote in [ruby-talk:198559]:

It’s disallowed. If it were possible, you could get an illegal
“String.” For instance:

[1].pack(‘l’).unpack(‘p*’)

Would it be possible to make an exception for strings (char pointers)?

How can you tell if it is a valid pointer? Use DL::Ptr if you
really sure, at your own risk.

Or is there no way to determine the type of underlying object in time?

Basically, no way to determine what a bare memory chunk is.