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Erlang Binaries Cheat Sheet by

Summarizing the syntax for handling binary data in Erlang.


Erlang can match binaries just as any list of things. <<E1, E2, E3>> = Bin divides the binary Bin into three elements of type integer of one byte each. This means that Bin has to be 24 bits long, or we get a badmatch. You can also make partial matches, in a [Head | Tail] fashion, by putting /bitstring on the last element, like so: <<E1, E2, E3/bit­str­ing­>> = Bin. This is a type modifier and tells Erlang that there are two 8-bit elements, in E1 and E2 respec­tively, and then an undete­rmined amount of bits stored in E3.

Type Modifiers

Size in bits
As many as it takes
Default size is 8 bits
Need to specify length if other than default: <<A­:16­/fl­oat­>>
8 per chunk
Anything matched must be of size evenly divisible by 8 (this is default)
1 per chunk
Will always match, use as Tail for a list
8-32, 16-32, and 32
<<"a­bc"/­utf­8>> is the same as <<$­a/utf8, $b/utf8, $c/utf­8>>
Default is unsigned
Endianness - native is resolved at load time to whatever the CPU uses
Define a custom unit of length 1..256


<<97, 98, 99>>
<<"a­bc">> (turn off with shell:­str­ing­s(f­alse))
<<A­:2/­unit:6, B:1/un­it:­4>> = <<7, 42>>
A = 114 B = 10
<<A­:16­/fl­oat­>> = <<1, 17>>
<<A­/si­gne­d>> = <<2­55>>
<<A­/bi­g>> = <<255, 0>>
<<A­/li­ttl­e>> = <<255, 0>>
How Erlang handles unicode
When constr­ucting a binary, if the size of an integer N is too large to fit inside the given segment, the most signif­icant bits are silently discarded and only the N least signif­icant bits kept.


Each segment in a binary has the following general syntax: Value:Size/TypeSpecifierList. The Size and TypeSpecifier can be omitted.

Value is either a literal or a variable, Size is multiplied by the unit in TypeSpecifierList, and can be any expression that evaluates to an integer. Think of 'Size' as the number of items of the type in the 'TypeSpecifierList'
Contrived example: <<X­:4/­lit­tle­-si­gne­d-i­nte­ger­-un­it:­8>> has a total size of 4*8 = 32 bits, and it contains a signed integer in little endian byte order.

Binary Compre­hension Example

Just like with lists, there is a notation for binary compre­hen­sion. Below is an example of how to use this to convert a 32 bit integer into a hex repres­ent­ation:

int_as­_he­x(Int) ->
 ­ ­ ­ ­Int­AsBin = <<I­nt:­32>­>,
 ­ ­ ­ ­"­0x" ++ lists:­fla­tte­n([­byt­e_t­o_h­ex(­<<B­yte­>>) || <<B­yte­:8>> <= IntAsB­in]).

byte_t­o_h­ex(­<<N­ibb­le1:4, Nibble­2:4­>>) ->
 ­ ­ ­ ­[in­teg­er_­to_­lis­t(N­ibble1, 16), intege­r_t­o_l­ist­(Ni­bble2, 16))].
You can mix list- and binary compre­hen­sion: if the generator is a list, use <-, if it's a binary, use <=. If you want the result to be a binary, use <<>>, if you want a list, use [] around the expres­sion.


Use the Erlang shell to trial and error you way to a correct expres­sion. A useful tool for unders­tanding why your binaries are badmat­ching is bit_size:
bit_si­ze(­<<1­/in­teg­er>­>). => 8 bit_si­ze(­<<<­<1:1, 0:1>>/­bit­str­ing­>>). => 2
bit_si­ze(­<<1.0/­flo­at>­>). => 64 bit_si­ze(­<<<­<1, 2>>­/bi­nar­y>>). => 16
A related one is byte_size:
MinByt­esT­oEn­cod­eNumber = byte_s­ize­(bi­nar­y:e­nco­de_­uns­ign­ed(­Num­ber)).


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