Type Modifiers
Type |
Size in bits |
Remarks |
|
As many as it takes |
Default size is 8 bits |
|
64|32|16 |
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>>
|
|
N/A |
Default is unsigned |
|
N/A |
Endianness - native
is resolved at load time to whatever the CPU uses |
|
N/A |
Define a custom unit of length 1..256 |
Examples
Expression |
Result |
|
<<"abc">>
(turn off with shell:strings(false)
) |
<<A:2/unit:6, B:1/unit:4>> = <<7, 42>>
|
|
<<A:16/float>> = <<1, 17>>
|
|
<<A/signed>> = <<255>>
|
|
<<A:16/big>> = <<255, 0>>
|
|
<<A:16/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.
|
|
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
1. 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. |
1 Mostly true, see Bit Syntax Expressions in Erlang documentation for complete picture.
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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