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

**URL:** <https://www.ruby-forum.com/t/using-pack-unpack-to-go-from-string-to-address-and-back-to-s/64040>\
**Category:** Ruby\
**Created:** [June 21, 2006, 4:24pm UTC](https://www.ruby-forum.com/t/using-pack-unpack-to-go-from-string-to-address-and-back-to-s/64040 "2006-06-21T16:24:06Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![BergerS\_Daniel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/bc8723/32.png) [@BergerS\_Daniel](https://www.ruby-forum.com/u/BergerS_Daniel)\
**Post date:** [June 21, 2006, 4:24pm UTC](https://www.ruby-forum.com/t/using-pack-unpack-to-go-from-string-to-address-and-back-to-s/64040/1 "2006-06-21T16:24:06Z")

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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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**Author:** ![BergerS\_Daniel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/bc8723/32.png) [@BergerS\_Daniel](https://www.ruby-forum.com/u/BergerS_Daniel)\
**Post date:** [June 22, 2006, 6:54pm UTC](https://www.ruby-forum.com/t/using-pack-unpack-to-go-from-string-to-address-and-back-to-s/64040/2 "2006-06-22T18:54:59Z")

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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\*’)

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<div class="post-metadata">

**Author:** ![BergerS\_Daniel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/bc8723/32.png) [@BergerS\_Daniel](https://www.ruby-forum.com/u/BergerS_Daniel)\
**Post date:** [June 22, 2006, 7:07pm UTC](https://www.ruby-forum.com/t/using-pack-unpack-to-go-from-string-to-address-and-back-to-s/64040/3 "2006-06-22T19:07:50Z")

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removed\_email\_address@domain.invalid 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

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<div class="post-metadata">

**Author:** ![BergerS\_Daniel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/bc8723/32.png) [@BergerS\_Daniel](https://www.ruby-forum.com/u/BergerS_Daniel)\
**Post date:** [June 23, 2006, 3:07am UTC](https://www.ruby-forum.com/t/using-pack-unpack-to-go-from-string-to-address-and-back-to-s/64040/4 "2006-06-23T03:07:00Z")

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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.