This section describes the "bit syntax" which was added to the Erlang language in release 5.0 (R7). Compared to the original bit syntax prototype by Claes Wikström and Tony Rogvall (presented on the Erlang User's Conference 1999), this implementation differs primarily in the following respects,
unit:U
has been added,
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 bytes. The purpose of a Bin is to be able to, from a high level, construct a binary,
Bin = <<E1, E2, ... En>>
in which case all elements must be bound, or to match a binary,
<<E1, E2, ... En>> = Bin
where Bin
is bound, and where the elements are bound or unbound,
as in any match.
Each element specifies a certain segment of the binary. A segment is 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 etc.
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">>
yields binaries of size 3; binary_to_list(Bin11)
evaluates to [1, 17, 42]
, and
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>>
yields a binary of size 4, and binary_to_list(Bin2)
evaluates to [1, 17, 00, 42]
too. Here we used a
size expression for the variable C
in order to
specify a 16-bits segment of Bin2
.
Example 3: A Bin can also be used for matching: if
D
, E
, and F
are unbound variables, and
Bin2
is bound as in the former example,
<<D:16, E, F/binary>> = Bin2
yields 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, where Dgram
is bound to the consecutive
bytes of an IP datagram of IP protocol version 4, and where we
want to extract the header and the data of the datagram:
-define(IP_VERSION, 4). -define(IP_MIN_HDR_LEN, 5). DgramSize = 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
should
bind to a binary. All other variables have the default type
equal to unsigned integer.
An IP datagram header is of variable length, and its length -
measured in the number of 32-bit words - is given in the segment
corresponding to HLen
, the minimum value of which is
5. It is the segment corresponding to Opts
that is
variable: if HLen
is equal to 5, Opts
will be an
empty binary.
The tail variables RestDgram
and Data
bind to
binaries, as all tail variables do. Both may bind to empty
binaries.
If the first 4-bits segment of Dgram
is not equal to
4, or if HLen
is less than 5, or if the size of
Dgram
is less than 4*HLen
, the match of
Dgram
fails.
Note 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
Both the Size
and the TypeSpecifier
or both may be
omitted; thus the following variations are allowed:
Value
Value:Size
Value/TypeSpecifierList
Default values will be used for missing specifications. The default values are described in the section "Defaults" below.
Used in binary construction, the Value
part is any expression.
Used in binary matching, the Value
part must be a literal or
variable. You can read more about the Value
part in the
sections about constructing binaries and matching binaries.
The Size
part of the segment multiplied by the unit in the
TypeSpecifierList
(described below) 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
, or binary
.
signed
or unsigned
. Note that signedness only matters for matching.
big
or
little
.
unit:IntegerLiteral
.
The allowed range is 1-256. It will be multiplied by the Size
specifier to give the effective size of the segment.
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 instance, 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 very last element. All other binary elements
in matching must have a size specification.
The default unit depends on the the type.
For integer
and float
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, but the total
number of bits in all segments must be evenly divisible by 8,
or in other words, the resulting binary must consist of a whole number
of bytes. An badarg
exception will be thrown if the resulting
binary is not byte-aligned. Example:
<<X:1,Y:6>>
The total number of bits is 7, which is not evenly divisible by 8;
thus, there will be badarg
exception (and a compiler warning
as well). The following example
<<X:1,Y:6,Z:1>>
will successfully construct a binary of 8 bits, or one byte. (Provided that all of X, Y and Z are integers.)
As noted 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 variable. The following gives a compiler syntax error:
<<X+1:8>>
This expression must be rewritten to
<<(X+1):8>>
in order to be accepted by the compiler.
As syntactic sugar, an literal string may be written instead of a element.
<<"hello">>
which is syntactic sugar for
<<$h,$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 binary pattern. A binary pattern can occur in every place patterns are allowed, also 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.
This means that the following head will never match:
foo(<<X:7/binary,Y:1/binary>>) ->
As noted earlier, segments have the following general syntax:
Value:Size/TypeSpecifierList
When matching Value
value must be either a variable or an integer
or floating point literal. Expressions are not allowed.
Size
must be an integer literal, or a previously bound variable.
Note that 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 like this:
foo(N, Bin) -> <<X:N,T/binary>> = Bin, {X,T}.
To match out the rest of binary, specify a binary field without size:
foo(<<A:8,Rest/binary>>) ->
As always, the size of the tail must be evenly divisible by 8.
Assume that we need a function that creates a binary out of a list of triples of integers. A first (inefficient) version of such a function could look like this:
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>>); % inefficient triples_to_bin([], Acc) -> Acc.
The reason for the inefficiency of this function is that for
each triple, the binary constructed so far (Acc
) is copied.
(Note: The original bit syntax prototype avoided the copy operation
by using segmented binaries, which are not implemented in R7.)
The efficient way to write this function in R7 is:
triples_to_bin(T) -> triples_to_bin(T, []). triples_to_bin([{X,Y,Z} | T], Acc) -> triples_to_bin(T, [<<X:32, Y:32, Z:32>> | Acc]); triples_to_bin([], Acc) -> list_to_binary(lists:reverse(Acc)).
Note that list_to_binary/1
handles deep lists of binaries
and small integers. (This fact was previously undocumented.)