OverviewErlang can match binaries just as any list of things. <<E1, E2, E3>> = Bin divides the binary Bin into three elements of type integer (which is default) 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 ModifiersType | Size in bits | Remarks | integer
| 8 | Default type | float
| 64 | binary|bytes
| 8 per chunk | Anything matched must be of size evenly divisible by 8 | 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>>
|
Other Type ModifiersKinds | Values | Remarks | Signedness | signed|unsigned
| Unsigned is default | Endianness | big|little|native
| native is resolved at load time to whatever the CPU uses
| Custom unit | unit:IntLiteral
| Define a custom unit of length 1..256 |
| | SegmentsEach segment in a binary has the following general syntax: Value:Size/TypeSpecifierList. The Size or the TypeSpecifier, or both, 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. |
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. ExamplesExpression | Result | <<"abc">>
| <<97,98,99>>
| <<1,17,42:16>>
| <<1,17,0,42>>
| <<A,B,C:16>> = <<1,17,42:16>>
| C = 42
| <<D:16,E,F>> = <<1,17,42:16>>
| D = 273 and F = 42
| <<G,Rest/binary>> = <<1,17,42:12>>
| <<1,17,0,42>>
| <<G,Rest/bitstring>> = <<1,17,42:12>>
| <<1,17,2,10:4>>
| <<IntAsLittleEndian>> =
<<IntAsBigEndian/little>>
| IntAsBigEndian = 9494 and
IntAsLittleEndian = 22
| <<"pöpcörn"/utf8>>
| This is 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. Binary Comprehension ExamplesJust like with lists, there is a notation for binary comprehension. It works almost the same, you just exchange the <- before the generator for a <=. Here 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))]. |
Depending on what you put around the comprehension expression, the end result is either a binary, or as above, a list. |
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