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authorErlang/OTP <[email protected]>2009-11-20 14:54:40 +0000
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+<?xml version="1.0" encoding="latin1" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+ <header>
+ <copyright>
+ <year>2000</year><year>2009</year>
+ <holder>Ericsson AB. All Rights Reserved.</holder>
+ </copyright>
+ <legalnotice>
+ The contents of this file are subject to the Erlang Public License,
+ Version 1.1, (the "License"); you may not use this file except in
+ compliance with the License. You should have received a copy of the
+ Erlang Public License along with this software. If not, it can be
+ retrieved online at http://www.erlang.org/.
+
+ Software distributed under the License is distributed on an "AS IS"
+ basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+ the License for the specific language governing rights and limitations
+ under the License.
+
+ </legalnotice>
+
+ <title>The Bit Syntax</title>
+ <prepared>Bj&ouml;rn Gustavsson</prepared>
+ <responsible>Bjarne D&auml;cker</responsible>
+ <docno>1</docno>
+ <approved>Bjarne D&auml;Ker</approved>
+ <checked></checked>
+ <date>00-06-21</date>
+ <rev>PA1</rev>
+ <file>bit_syntax.sgml</file>
+ </header>
+ <p>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 Wikstr&ouml;m and Tony Rogvall (presented on the
+ Erlang User's Conference 1999), this implementation differs
+ primarily in the following respects,
+ </p>
+ <list type="ordered">
+ <item>
+ <p>the character pairs '&lt;&lt;' and '&gt;&gt;' are used to delimit
+ a binary patterns and constructor (not '&lt;' and '&gt;' as in
+ the prototype),
+ </p>
+ </item>
+ <item>
+ <p>the tail syntax ('|Variable') has been eliminated,
+ </p>
+ </item>
+ <item>
+ <p>all size expressions must be bound,
+ </p>
+ </item>
+ <item>
+ <p>a type <c>unit:U</c> has been added,
+ </p>
+ </item>
+ <item>
+ <p>lists and tuples cannot be generated
+ </p>
+ </item>
+ <item>
+ <p>there are no paddings whatsoever.
+ </p>
+ </item>
+ </list>
+
+ <section>
+ <title>Introduction</title>
+ <p>In Erlang a Bin is used for constructing binaries and
+ matching binary patterns. A Bin is written with the
+ following syntax:</p>
+ <code type="none"><![CDATA[
+ <<E1, E2, ... En>>
+ ]]></code>
+ <p>A Bin is a low-level sequence of bytes. The purpose of a
+ Bin is to be able to, from a high level,
+ <em>construct</em> a binary,
+ </p>
+ <code type="none"><![CDATA[
+ Bin = <<E1, E2, ... En>>
+ ]]></code>
+ <p>in which case all elements must be bound, or to
+ <em>match</em> a binary,
+ </p>
+ <code type="none"><![CDATA[
+ <<E1, E2, ... En>> = Bin
+ ]]></code>
+ <p>where <c>Bin</c> is bound, and where the elements are bound or unbound,
+ as in any match.
+ </p>
+ <p>Each element specifies a certain <em>segment</em> 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.
+ </p>
+ <p>The following examples illustrate how binaries are constructed
+ or matched, and how elements and tails are specified.
+ </p>
+
+ <section>
+ <title>Examples</title>
+ <p><em>Example 1: </em>A binary can be constructed from a set of
+ constants or a string literal:</p>
+ <code type="none"><![CDATA[
+ Bin11 = <<1, 17, 42>>,
+ Bin12 = <<"abc">>
+ ]]></code>
+ <p>yields binaries of size 3; <c>binary_to_list(Bin11)</c>
+ evaluates to <c>[1, 17, 42]</c>, and
+ <c>binary_to_list(Bin12)</c> evaluates to <c>[97, 98, 99]</c>.
+ </p>
+ <p><em>Example 2: </em>Similarly, a binary can be constructed
+ from a set of bound variables:</p>
+ <code type="none"><![CDATA[
+ A = 1, B = 17, C = 42,
+ Bin2 = <<A, B, C:16>>
+ ]]></code>
+ <p>yields a binary of size 4, and <c>binary_to_list(Bin2)</c>
+ evaluates to <c>[1, 17, 00, 42]</c> too. Here we used a
+ <em>size expression</em> for the variable <c>C</c> in order to
+ specify a 16-bits segment of <c>Bin2</c>.
+ </p>
+ <p><em>Example 3: </em>A Bin can also be used for matching: if
+ <c>D</c>, <c>E</c>, and <c>F</c> are unbound variables, and
+ <c>Bin2</c> is bound as in the former example,</p>
+ <code type="none"><![CDATA[
+ <<D:16, E, F/binary>> = Bin2
+ ]]></code>
+ <p>yields <c>D = 273</c>, <c>E = 00</c>, and F binds to a binary
+ of size 1: <c>binary_to_list(F) = [42]</c>.
+ </p>
+ <p><em>Example 4: </em>The following is a more elaborate example
+ of matching, where <c>Dgram</c> 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:</p>
+ <code type="none"><![CDATA[
+ -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.
+ ]]></code>
+ <p>Here the segment corresponding to the <c>Opts</c> variable
+ has a <em>type modifier</em> specifying that <c>Opts</c> should
+ bind to a binary. All other variables have the default type
+ equal to unsigned integer.
+ </p>
+ <p>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 <c>HLen</c>, the minimum value of which is
+ 5. It is the segment corresponding to <c>Opts</c> that is
+ variable: if <c>HLen</c> is equal to 5, <c>Opts</c> will be an
+ empty binary.
+ </p>
+ <p>The tail variables <c>RestDgram</c> and <c>Data</c> bind to
+ binaries, as all tail variables do. Both may bind to empty
+ binaries.
+ </p>
+ <p>If the first 4-bits segment of <c>Dgram</c> is not equal to
+ 4, or if <c>HLen</c> is less than 5, or if the size of
+ <c>Dgram</c> is less than <c>4*HLen</c>, the match of
+ <c>Dgram</c> fails.
+ </p>
+ </section>
+ </section>
+
+ <section>
+ <title>A Lexical Note</title>
+ <p>Note that "<c><![CDATA[B=<<1>>]]></c>" will be interpreted as
+ "<c><![CDATA[B =< ;<1>>]]></c>", which is a syntax error.
+ The correct way to write the expression is "<c><![CDATA[B = <<1>>]]></c>".</p>
+ </section>
+
+ <section>
+ <title>Segments</title>
+ <p>Each segment has the following general syntax:</p>
+ <p><c>Value:Size/TypeSpecifierList</c></p>
+ <p>Both the <c>Size</c> and the <c>TypeSpecifier</c> or both may be
+ omitted; thus the following variations are allowed:
+ </p>
+ <p><c>Value</c></p>
+ <p><c>Value:Size</c></p>
+ <p><c>Value/TypeSpecifierList</c></p>
+ <p>Default values will be used for missing specifications. The default
+ values are described in the section "Defaults" below.
+ </p>
+ <p>Used in binary construction, the <c>Value</c> part is any expression.
+ Used in binary matching, the <c>Value</c> part must be a literal or
+ variable. You can read more about the <c>Value</c> part in the
+ sections about constructing binaries and matching binaries.
+ </p>
+ <p>The <c>Size</c> part of the segment multiplied by the unit in the
+ <c>TypeSpecifierList</c> (described below) gives the number of bits
+ for the segment. In construction, <c>Size</c> is any expression that
+ evaluates to an integer. In matching, <c>Size</c> must be a constant
+ expression or a variable.
+ </p>
+ <p>The <c>TypeSpecifierList</c> is a list of type specifiers separated by
+ hyphens.
+ </p>
+ <taglist>
+ <tag>Type</tag>
+ <item>The type can be <c>integer</c>, <c>float</c>, or <c>binary</c>.</item>
+ <tag>Signedness</tag>
+ <item>The signedness specification can be either <c>signed</c>
+ or <c>unsigned</c>. Note that signedness only matters for matching.</item>
+ <tag>Endianness</tag>
+ <item>The endianness specification can be either <c>big</c>,
+ <c>little</c>, or <c>native</c>. Native-endian means that
+ the endian will be 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.</item>
+ <tag>Unit</tag>
+ <item>The unit size is given as <c>unit:IntegerLiteral</c>.
+ The allowed range is 1-256. It will be multiplied by the <c>Size</c>
+ specifier to give the effective size of the segment.</item>
+ </taglist>
+ <p>Example:
+ </p>
+ <code type="none">
+X:4/little-signed-integer-unit:8
+ </code>
+ <p>This element has a total size of 4*8 = 32 bits, and it contains a
+ signed integer in little-endian order.</p>
+ </section>
+
+ <section>
+ <title>Defaults</title>
+ <p>The default type for a segment is <c>integer</c>. The default type
+ does <em>not</em> depend on the value, even if the value is a literal.
+ For instance, the default type in '<c><![CDATA[<<3.14>>]]></c>' is <c>integer</c>,
+ not <c>float</c>.
+ </p>
+ <p>The default <c>Size</c> depends on the type.
+ For <c>integer</c> it is 8. For <c>float</c> it is 64.
+ For <c>binary</c> 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.
+ </p>
+ <p>The default unit depends on the the type.
+ For <c>integer</c> and <c>float</c> it is 1.
+ For <c>binary</c> it is 8.
+ </p>
+ <p>The default signedness is <c>unsigned</c>.
+ </p>
+ <p>The default endianness is <c>big</c>.</p>
+ </section>
+
+ <section>
+ <title>Constructing Binaries</title>
+ <p>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 <c>badarg</c> exception.
+ </p>
+ <p>There can be zero or more segments in a binary to be constructed.
+ The expression '<c><![CDATA[<<>>]]></c>' constructs a zero length binary.
+ </p>
+ <p>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 <c>badarg</c> exception will be thrown if the resulting
+ binary is not byte-aligned. Example:
+ </p>
+ <code type="none"><![CDATA[
+<<X:1,Y:6>>
+ ]]></code>
+ <p>The total number of bits is 7, which is not evenly divisible by 8;
+ thus, there will be <c>badarg</c> exception (and a compiler warning
+ as well). The following example
+ </p>
+ <code type="none"><![CDATA[
+<<X:1,Y:6,Z:1>>
+ ]]></code>
+ <p>will successfully construct a binary of 8 bits, or one byte. (Provided
+ that all of X, Y and Z are integers.)
+ </p>
+ <p>As noted earlier, segments have the following general syntax:
+ </p>
+ <p><c>Value:Size/TypeSpecifierList</c></p>
+ <p>When constructing binaries, <c>Value</c> and <c>Size</c> can be
+ any Erlang expression. However, for syntactical reasons,
+ both <c>Value</c> and <c>Size</c> 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:
+ </p>
+ <code type="none"><![CDATA[
+<<X+1:8>>
+ ]]></code>
+ <p>This expression must be rewritten to
+ </p>
+ <code type="none"><![CDATA[
+<<(X+1):8>>
+ ]]></code>
+ <p>in order to be accepted by the compiler.
+ </p>
+
+ <section>
+ <title>Including Literal Strings</title>
+ <p>As syntactic sugar, an literal string may be written instead of a
+ element.</p>
+ <code type="none"><![CDATA[
+<<"hello">> ]]></code>
+ <p>which is syntactic sugar for</p>
+ <code type="none"><![CDATA[
+<<$h,$e,$l,$l,$o>> ]]></code>
+ </section>
+ </section>
+
+ <section>
+ <title>Matching Binaries</title>
+ <p>This section describes the rules for matching binaries using the
+ bit syntax.
+ </p>
+ <p>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.
+ </p>
+ <p>The pattern '<c><![CDATA[<<>>]]></c>' matches a zero length binary.
+ </p>
+ <p>Each segment in a binary can consist of zero or more bits.
+ </p>
+ <p>A segment of type <c>binary</c> must have a size evenly divisible by 8.
+ </p>
+ <p>This means that the following head will never match:</p>
+ <code type="none"><![CDATA[
+foo(<<X:7/binary,Y:1/binary>>) -> ]]></code>
+ <p>As noted earlier, segments have the following general syntax:
+ </p>
+ <p><c>Value:Size/TypeSpecifierList</c></p>
+ <p>When matching <c>Value</c> value must be either a variable or an integer
+ or floating point literal. Expressions are not allowed.
+ </p>
+ <p><c>Size</c> must be an integer literal, or a previously bound variable.
+ Note that the following is not allowed:</p>
+ <code type="none"><![CDATA[
+foo(N, <<X:N,T/binary>>) ->
+ {X,T}. ]]></code>
+ <p>The two occurrences of <c>N</c> are not related. The compiler
+ will complain that the <c>N</c> in the size field is unbound.
+ </p>
+ <p>The correct way to write this example is like this:</p>
+ <code type="none"><![CDATA[
+foo(N, Bin) ->
+ <<X:N,T/binary>> = Bin,
+ {X,T}. ]]></code>
+
+ <section>
+ <title>Getting the Rest of the Binary</title>
+ <p>To match out the rest of binary, specify a binary field without size:</p>
+ <code type="none"><![CDATA[
+foo(<<A:8,Rest/binary>>) -> ]]></code>
+ <p>As always, the size of the tail must be evenly divisible by 8.
+ </p>
+ </section>
+ </section>
+
+ <section>
+ <title>Traps and Pitfalls</title>
+ <p>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:</p>
+ <code type="none"><![CDATA[
+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. ]]></code>
+ <p>The reason for the inefficiency of this function is that for
+ each triple, the binary constructed so far (<c>Acc</c>) is copied.
+ (Note: The original bit syntax prototype avoided the copy operation
+ by using segmented binaries, which are not implemented in R7.)
+ </p>
+ <p>The efficient way to write this function in R7 is:</p>
+ <code type="none"><![CDATA[
+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)). ]]></code>
+ <p>Note that <c>list_to_binary/1</c> handles deep lists of binaries
+ and small integers. (This fact was previously undocumented.)
+ </p>
+ </section>
+</chapter>
+