Type Modifiers
Type |
Size in bits |
Remarks |
integer
|
As many as it takes |
Default size is 8 bits |
float
|
64|32|16 |
Need to specify length if other than default: <<A:16/float>> |
binary|bytes
|
8 per chunk |
Anything matched must be of size evenly divisible by 8 (this is default) |
bitstring|bits
|
1 per chunk |
Will always match, use as Tail for a list |
utf8|utf16|utf32
|
8-32, 16-32, and 32 |
<<"abc"/utf8>> is the same as <<$a/utf8, $b/utf8, $c/utf8>>
|
signed|unsigned
|
N/A |
Default is unsigned |
big|little|native
|
N/A |
Endianness - native is resolved at load time to whatever the CPU uses |
unit:IntLiteral
|
N/A |
Define a custom unit of length 1..256 |
Examples
Expression |
Result |
<<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>>
|
1.627206802368164e-5
|
<<A/signed>> = <<255>>
|
-1
|
<<A/big>> = <<255, 0>>
|
255
|
<<A/little>> = <<255, 0>>
|
65280
|
<<"pöpcörn"/utf8>>
|
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.
|
|
Segments
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.
Troubleshooting
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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