[erlang-questions] Terms to binary and viceversa

Fred Hebert <>
Thu Jan 17 01:38:33 CET 2013


I went with whatever structure was mentionned in the parent post.

If the pid expected is 27 bytes then it should work, if for example it
equals some hardcoded node name (which is a big no-no to begin with.)
Personally I'd use the Erlang External Term Format
(http://erlang.org/doc/apps/erts/erl_ext_dist.html) (or BERT) and
serialize everything with it, or go with a prefix indicating the length
of the term to follow to make it simpler.

On 01/17, Bj�rn-Egil Dahlberg wrote:
> Pids are not always 27 bytes. Names are atoms.
> 
> Example:
> 
> -module(t).
> 
> -compile([export_all]).
> 
> start() ->
>     Pid = self(),
>     PidBin = term_to_binary(Pid),
>     SomeData = <<"some other data">>,
>     Bin = <<$A, PidBin/binary, SomeData/binary>>,
>     <<$A, 131, 103, 100, Len:16, Name:Len/binary, Id:4/binary,
> Serial:4/binary, Creation, Rest/binary>> = Bin,
>     DecPid = binary_to_term(<<131, 103, 100, Len:16, Name/binary,
> Id/binary, Serial/binary, Creation>>),
>     {Pid, DecPid, Rest}.
> 
> Output:
> 
>  ~ $ erl -sname some-node-name-used
> Erlang R15B (erts-5.9) [source] [smp:4:4] [async-threads:0] [hipe]
> [kernel-poll:false]
> 
> Eshell V5.9  (abort with ^G)
> ()1> t:start().
> {<0.37.0>,<0.37.0>,<<"some other data">>}
> 
> 
> // Björn-Egil
> 
> 
> 2013/1/17 Fred Hebert <>
> 
> > The pattern:
> >
> >     <<$A, BinPid:27/binary, Data/binary>>
> >
> > Should do it. Then you can get `Pid` by calling `binary_to_term/1` on
> > `BinPid`. By default, the size of the `binary` type of matching is in
> > bytes, so you should be extracting it fine. To get it in bits, you'd
> > need to specify `BinPid:216/binary-unit:1`, for example.
> >
> > In your case, what you were doing is returning `Pid` as a 216 bits
> > integer (the default type), not something you can use in any useful way.
> >
> > Regards,
> > Fred.
> >
> > On 01/17, Alessandro Sivieri wrote:
> > > Hi all,
> > >
> > > I have the following situation: two processes are exchanging data, and
> > one
> > > particular packet contains the following three contents:
> > >
> > >    - a character (which defines the type of that specific packet, in this
> > >    case it is a 'A')
> > >    - a pid
> > >    - some binary data which I encode and decode in a specific way
> > >
> > > The question is: how can I extract the pid part using the bit syntax? I
> > am
> > > able to extract the character, and I know that the pid (encoded to binary
> > > with term_to_binary) is 27 bytes, or 216 bits, so my first thought was
> > > something like
> > >
> > > <<$A:8, Pid:216, Data/binary>>
> > >
> > > But this is wrong because in Pid at this point I have a very large
> > integer
> > > (and I see my error in that pattern), so do I have to get each single
> > byte
> > > of the 27 bytes composing the pid one by one or is there a pattern that
> > > immediately matches that term giving me something that I can pass to
> > > binary_to_term? I have experimented a little but I always obtain a no
> > > matching clause...
> > >
> > > Alessandro
> > >
> > > --
> > > Sivieri Alessandro
> > > 
> > > http://sivieri.wordpress.com/
> >
> > > _______________________________________________
> > > erlang-questions mailing list
> > > 
> > > http://erlang.org/mailman/listinfo/erlang-questions
> >
> > _______________________________________________
> > erlang-questions mailing list
> > 
> > http://erlang.org/mailman/listinfo/erlang-questions
> >



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