The complete specification for the bit syntax appears in the Reference Manual
.
In Erlang, a Bin is used for constructing binaries and matching binary patterns. A Bin is written with the following syntax:
<<E1, E2, ... En>>
A Bin is a low-level sequence of bits or bytes. The purpose of a Bin is to enable construction of binaries:
Bin = <<E1, E2, ... En>>
All elements must be bound. Or match a binary:
<<E1, E2, ... En>> = Bin
Here, Bin
is bound and the elements are bound or unbound, as in any match.
A Bin does not need to consist of a whole number of bytes.
A bitstring is a sequence of zero or more bits, where the number of bits does not need to be divisible by 8. If the number of bits is divisible by 8, the bitstring is also a binary.
Each element specifies a certain segment of the bitstring. A segment is a set of contiguous bits of the binary (not necessarily on a byte boundary). The first element specifies the initial segment, the second element specifies the following segment, and so on.
The following examples illustrate how binaries are constructed, or matched, and how elements and tails are specified.
Example 1: A binary can be constructed from a set of constants or a string literal:
Bin11 = <<1, 17, 42>>, Bin12 = <<"abc">>
This gives two binaries of size 3, with the following evaluations:
binary_to_list(Bin11)
evaluates to [1, 17, 42]
.binary_to_list(Bin12)
evaluates to [97, 98, 99]
.Example 2:Similarly, a binary can be constructed from a set of bound variables:
A = 1, B = 17, C = 42, Bin2 = <<A, B, C:16>>
This gives a binary of size 4. Here, a size expression is used for the variable C
to specify a 16-bits segment of Bin2
.
binary_to_list(Bin2)
evaluates to [1, 17, 00, 42]
.
Example 3: A Bin can also be used for matching. D
, E
, and F
are unbound variables, and Bin2
is bound, as in Example 2:
<<D:16, E, F/binary>> = Bin2
This gives D = 273
, E = 00
, and F binds to a binary of size 1: binary_to_list(F) = [42]
.
Example 4: The following is a more elaborate example of matching. Here, Dgram
is bound to the consecutive bytes of an IP datagram of IP protocol version 4. The ambition is to extract the header and the data of the datagram:
-define(IP_VERSION, 4). -define(IP_MIN_HDR_LEN, 5). DgramSize = byte_size(Dgram), case Dgram of <<?IP_VERSION:4, HLen:4, SrvcType:8, TotLen:16, ID:16, Flgs:3, FragOff:13, TTL:8, Proto:8, HdrChkSum:16, SrcIP:32, DestIP:32, RestDgram/binary>> when HLen>=5, 4*HLen=<DgramSize -> OptsLen = 4*(HLen - ?IP_MIN_HDR_LEN), <<Opts:OptsLen/binary,Data/binary>> = RestDgram, ... end.
Here, the segment corresponding to the Opts
variable has a type modifier, specifying that Opts
is to bind to a binary. All other variables have the default type equal to unsigned integer.
An IP datagram header is of variable length. This length is measured in the number of 32-bit words and is given in the segment corresponding to HLen
. The minimum value of HLen
is 5. It is the segment corresponding to Opts
that is variable, so if HLen
is equal to 5, Opts
becomes an empty binary.
The tail variables RestDgram
and Data
bind to binaries, as all tail variables do. Both can bind to empty binaries.
The match of Dgram
fails if one of the following occurs:
Dgram
is not equal to 4.HLen
is less than 5.Dgram
is less than 4*HLen
.Notice that "B=<<1>>
" will be interpreted as "B =< <1>>
", which is a syntax error. The correct way to write the expression is: B = <<1>>
.
Each segment has the following general syntax:
Value:Size/TypeSpecifierList
The Size
or the TypeSpecifier
, or both, can be omitted. Thus, the following variants are allowed:
Value
Value:Size
Value/TypeSpecifierList
Default values are used when specifications are missing. The default values are described in Defaults
.
The Value
part is any expression, when used in binary construction. Used in binary matching, the Value
part must be a literal or a variable. For more information about the Value
part, see Constructing Binaries and Bitstrings
and Matching Binaries
.
The Size
part of the segment multiplied by the unit in TypeSpecifierList
(described later) gives the number of bits for the segment. In construction, Size
is any expression that evaluates to an integer. In matching, Size
must be a constant expression or a variable.
The TypeSpecifierList
is a list of type specifiers separated by hyphens.
integer
, float
, and binary
. See Bit Syntax Expressions in the Reference Manual
for a complete description. signed
or unsigned
. Notice that signedness only matters for matching.big
, little
, or native
. Native-endian means that the endian is resolved at load time, to be either big-endian or little-endian, depending on what is "native" for the CPU that the Erlang machine is run on.unit:IntegerLiteral
. The allowed range is 1-256. It is multiplied by the Size
specifier to give the effective size of the segment. The unit size specifies the alignment for binary segments without size.Example:
X:4/little-signed-integer-unit:8
This element has a total size of 4*8 = 32 bits, and it contains a signed integer in little-endian order.
The default type for a segment is integer. The default type does not depend on the value, even if the value is a literal. For example, the default type in <<3.14>>
is integer, not float.
The default Size
depends on the type. For integer it is 8. For float it is 64. For binary it is all of the binary. In matching, this default value is only valid for the last element. All other binary elements in matching must have a size specification.
The default unit depends on the the type. For integer
, float
, and bitstring
it is 1. For binary it is 8.
The default signedness is unsigned
.
The default endianness is big
.
This section describes the rules for constructing binaries using the bit syntax. Unlike when constructing lists or tuples, the construction of a binary can fail with a badarg
exception.
There can be zero or more segments in a binary to be constructed. The expression <<>>
constructs a zero length binary.
Each segment in a binary can consist of zero or more bits. There are no alignment rules for individual segments of type integer
and float
. For binaries and bitstrings without size, the unit specifies the alignment. Since the default alignment for the binary
type is 8, the size of a binary segment must be a multiple of 8 bits, that is, only whole bytes.
Example:
<<Bin/binary,Bitstring/bitstring>>
The variable Bin
must contain a whole number of bytes, because the binary
type defaults to unit:8
. A badarg
exception is generated if Bin
consist of, for example, 17 bits.
The Bitstring
variable can consist of any number of bits, for example, 0, 1, 8, 11, 17, 42, and so on. This is because the default unit
for bitstrings is 1.
For clarity, it is recommended not to change the unit size for binaries. Instead, use binary
when you need byte alignment and bitstring
when you need bit alignment.
The following example successfully constructs a bitstring of 7 bits, provided that all of X and Y are integers:
<<X:1,Y:6>>
As mentioned earlier, segments have the following general syntax:
Value:Size/TypeSpecifierList
When constructing binaries, Value
and Size
can be any Erlang expression. However, for syntactical reasons, both Value
and Size
must be enclosed in parenthesis if the expression consists of anything more than a single literal or a variable. The following gives a compiler syntax error:
<<X+1:8>>
This expression must be rewritten into the following, to be accepted by the compiler:
<<(X+1):8>>
A literal string can be written instead of an element:
<<"hello">>
This is syntactic sugar for the following:
<<$h,$e,$l,$l,$o>>
This section describes the rules for matching binaries, using the bit syntax.
There can be zero or more segments in a binary pattern. A binary pattern can occur wherever patterns are allowed, including inside other patterns. Binary patterns cannot be nested. The pattern <<>>
matches a zero length binary.
Each segment in a binary can consist of zero or more bits. A segment of type binary
must have a size evenly divisible by 8 (or divisible by the unit size, if the unit size has been changed). A segment of type bitstring
has no restrictions on the size.
As mentioned earlier, segments have the following general syntax:
Value:Size/TypeSpecifierList
When matching Value
, value must be either a variable or an integer, or a floating point literal. Expressions are not allowed.
Size
must be an integer literal, or a previously bound variable. The following is not allowed:
foo(N, <<X:N,T/binary>>) -> {X,T}.
The two occurrences of N
are not related. The compiler will complain that the N
in the size field is unbound.
The correct way to write this example is as follows:
foo(N, Bin) -> <<X:N,T/binary>> = Bin, {X,T}.
To match out the rest of a binary, specify a binary field without size:
foo(<<A:8,Rest/binary>>) ->
The size of the tail must be evenly divisible by 8.
To match out the rest of a bitstring, specify a field without size:
foo(<<A:8,Rest/bitstring>>) ->
There are no restrictions on the number of bits in the tail.
Appending to a binary in an efficient way can be done as follows:
triples_to_bin(T) -> triples_to_bin(T, <<>>). triples_to_bin([{X,Y,Z} | T], Acc) -> triples_to_bin(T, <<Acc/binary,X:32,Y:32,Z:32>>); triples_to_bin([], Acc) -> Acc.
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Licensed under the Apache License, Version 2.0.