[erlang-questions] Bit Packing question

Erik Søe Sørensen <>
Sun Dec 14 13:04:51 CET 2014


Since no-one else mentioned it:
What you're looking for is probably the "unit" modifier:
1> Num = 123.
123
2> <<Num:4/unit:8>>.
<<0,0,0,123>>

The default unit for (integers/floats/bitstring) is 1 bit, while the
default unit for binaries ('bytes') is 1 byte.

/Erik

2014-12-13 15:35 GMT+01:00 PAILLEAU Eric <>:
>
> 5> A = <<1,2,8>>.
> <<1,2,8>>
> 6> P = <<A:2/binary>>.
> <<1,2>>
>
> Got it ?
>
> Le 13/12/2014 15:32, PAILLEAU Eric a écrit :
>
>  1> Num = 128.
>> 128
>> 2> BNum = <<Num>>.
>> <<128>>
>> 3>  Packet = <<BNum:1/binary>>.
>> <<128>>
>>
>> regards.
>>
>>
>> Le 13/12/2014 15:25, Santosh Kumar a écrit :
>>
>>> Hi Wes,
>>> I just tried <<BNum:4/binary>> and that didn't work as well. Here are
>>> the complete steps I ran through in erl:
>>>
>>> 1> Num = 128.
>>>
>>> 128
>>>
>>> 2> BNum = <<Num>>.
>>>
>>> <<128>>
>>>
>>> 3> Packet = <<BNum:4/binary>>.
>>>
>>> ** exception error: bad argument
>>>
>>>
>>> Any help much appreciated.
>>>
>>>
>>> Thanks
>>>
>>> Santosh
>>>
>>>
>>> On Fri, Dec 12, 2014 at 11:54 PM, Wes James <
>>> <mailto:>> wrote:
>>>
>>>     According to:
>>>
>>>     http://www.erlang.org/doc/programming_examples/bit_syntax.html
>>>
>>>     after the / you need integer, float or binary, i.e.,
>>> <<BNum:4/binary>>
>>>
>>>     -wes
>>>
>>>
>>>     On Fri Dec 12 2014 at 9:22:59 PM Santosh Kumar <
>>>     <mailto:>> wrote:
>>>
>>>         Hrm, I'm not sure I understand. Here's some code:
>>>
>>>         Num = 128.
>>>
>>>         128
>>>
>>>
>>>         So, Num is a number. I then do:
>>>
>>>         BNum = <<Num>>.
>>>
>>>
>>>         Now, BNum is a binary. I check it too:
>>>
>>>         is_binary(BNum).
>>>
>>>         true
>>>
>>>
>>>         I now, try doing:
>>>
>>>         Packet = <<BNum:4/bytes>>.
>>>
>>>         ** exception error: bad argument
>>>
>>>
>>>         Here, I am trying to create another binary Packet, that is 4
>>>         bytes long as and has as it's contents BNum. If what you say is
>>>         true and "/bytes" is used to pack Binary data this would work.
>>>         Unless I am missing something.
>>>
>>>
>>>         Thanks for your response.
>>>
>>>
>>>         Santosh
>>>
>>>
>>>         On Fri, Dec 12, 2014 at 10:39 PM, Bob Ippolito <
>>>         <mailto:>> wrote:
>>>
>>>             'bytes' is for packing binaries, not numbers. The
>>>             working example you have is the correct way to do it.
>>>
>>>
>>>             On Friday, December 12, 2014, Santosh Kumar
>>>             < <mailto:>> wrote:
>>>
>>>                 Hi group,
>>>                 I was trying to pack a number into 4 bytes and was
>>>                 having a hard time doing it. Here is what I was trying:
>>>
>>>                 102> Num = 128.
>>>
>>>                 128
>>>
>>>                 103> <<Num:4/bytes>>.
>>>
>>>                 ** exception error: bad argument
>>>
>>>
>>>                 While this works:
>>>
>>>
>>>                 104> <<Num:32>>.
>>>
>>>                 <<0,0,0,128>>
>>>
>>>
>>>                 Here the packing into 32 bits works but the packing into
>>>                 4 bytes fails. Not sure why.
>>>
>>>
>>>                 Any help, much appreciated.
>>>
>>>
>>>                 Thanks
>>>
>>>                 Santosh
>>>
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