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diff --git a/system/doc/programming_examples/bit_syntax.xml b/system/doc/programming_examples/bit_syntax.xml index fb321c1ba9..7ede5b71f9 100644 --- a/system/doc/programming_examples/bit_syntax.xml +++ b/system/doc/programming_examples/bit_syntax.xml @@ -31,62 +31,64 @@ <section> <title>Introduction</title> - <p>In Erlang a Bin is used for constructing binaries and matching + <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 bits or bytes. The purpose of a Bin is - to be able to, from a high level, construct a binary,</p> + <p>A Bin is a low-level sequence of bits or bytes. + The purpose of a Bin is to enable construction of binaries:</p> <code type="none"><![CDATA[ Bin = <<E1, E2, ... En>>]]></code> - <p>in which case all elements must be bound, or to match a binary,</p> + <p>All elements must be bound. Or match 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 + <p>Here, <c>Bin</c> is bound and the elements are bound or unbound, as in any match.</p> - <p>In R12B, a Bin need not consist of a whole number of bytes.</p> + <p>Since Erlang R12B, a Bin does not need to consist of a whole number of bytes.</p> <p>A <em>bitstring</em> is a sequence of zero or more bits, where - the number of bits doesn't need to be divisible by 8. If the number + 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.</p> <p>Each element specifies a certain <em>segment</em> 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 etc.</p> - <p>The following examples illustrate how binaries are constructed + segment, and so on.</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 + <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 + <p>This gives two binaries of size 3, with the following evaluations:</p> + <list type="bulleted"> + <item><c>binary_to_list(Bin11)</c> evaluates to <c>[1, 17, 42]</c>.</item> + <item><c>binary_to_list(Bin12)</c> evaluates to <c>[97, 98, 99]</c>.</item> + </list> + <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 + <p>This gives a binary of size 4. + Here, a <em>size expression</em> is used for the variable <c>C</c> 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 + <p><c>binary_to_list(Bin2)</c> evaluates to <c>[1, 17, 00, 42]</c>.</p> + <p><em>Example 3:</em> A Bin can also be used for matching. <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> + <c>Bin2</c> is bound, as in Example 2:</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 + <p>This gives <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> + of matching. Here, <c>Dgram</c> 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:</p> <code type="none"><![CDATA[ -define(IP_VERSION, 4). -define(IP_MIN_HDR_LEN, 5). @@ -102,53 +104,59 @@ case Dgram of <<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 + <p>Here, the segment corresponding to the <c>Opts</c> variable + has a <em>type modifier</em>, specifying that <c>Opts</c> is to 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>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 <c>HLen</c>. The minimum value of + <c>HLen</c> is 5. It is the segment corresponding to <c>Opts</c> + that is variable, so if <c>HLen</c> is equal to 5, <c>Opts</c> + becomes 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, as all tail variables do. Both can 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> + <p>The match of <c>Dgram</c> fails if one of the following occurs:</p> + <list type="bulleted"> + <item>The first 4-bits segment of <c>Dgram</c> is not equal to 4.</item> + <item><c>HLen</c> is less than 5.</item> + <item>The size of <c>Dgram</c> is less than <c>4*HLen</c>.</item> + </list> </section> </section> <section> - <title>A Lexical Note</title> - <p>Note that "<c><![CDATA[B=<<1>>]]></c>" will be interpreted as + <title>Lexical Note</title> + <p>Notice 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> + 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 + <p>The <c>Size</c> or the <c>TypeSpecifier</c>, or both, can be + omitted. Thus, the following variants are allowed:</p> + <list type="bulleted"> + <item><c>Value</c></item> + <item><c>Value:Size</c></item> + <item><c>Value/TypeSpecifierList</c></item> + </list> + <p>Default values are used when specifications are missing. + The default values are described in <seealso marker="#Defaults">Defaults</seealso>.</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 section about constructing - binaries and matching binaries.</p> + <p>The <c>Value</c> part is any expression, when used in binary construction. + Used in binary matching, the <c>Value</c> part must + be a literal or a variable. For more information about + the <c>Value</c> part, see + <seealso marker="#Constructing Binaries and Bitstrings">Constructing Binaries and Bitstrings</seealso> + and + <seealso marker="#Matching Binaries">Matching Binaries</seealso>.</p> <p>The <c>Size</c> part of the segment multiplied by the unit in - the <c>TypeSpecifierList</c> (described below) gives the number + <c>TypeSpecifierList</c> (described later) 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> @@ -160,22 +168,22 @@ end.]]></code> <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 + or <c>unsigned</c>. Notice 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 + 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.</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 allowed range is 1-256. It is multiplied by the <c>Size</c> specifier to give the effective size of - the segment. In R12B, the unit size specifies the alignment - for binary segments without size (examples will follow).</item> + the segment. Since Erlang R12B, the unit size specifies the alignment + for binary segments without size.</item> </taglist> - <p>Example:</p> + <p><em>Example:</em></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 @@ -184,13 +192,14 @@ X:4/little-signed-integer-unit:8</code> <section> <title>Defaults</title> - <p><marker id="Defaults"></marker>The default type for a segment is integer. The default + <p><marker id="Defaults"></marker>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 '<c><![CDATA[<<3.14>>]]></c>' is + literal. For example, the default type in <c><![CDATA[<<3.14>>]]></c> is integer, not float.</p> <p>The default <c>Size</c> 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 + matching, this default value is only valid for the 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>, @@ -201,61 +210,60 @@ X:4/little-signed-integer-unit:8</code> <section> <title>Constructing Binaries and Bitstrings</title> + <marker id="Constructing Binaries and Bitstrings"></marker> <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 + 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 of type <c>integer</c> and <c>float</c>. For binaries and bitstrings without size, the unit specifies the alignment. Since the default alignment for the <c>binary</c> type is 8, the size of a binary - segment must be a multiple of 8 bits (i.e. only whole bytes). - Example:</p> + segment must be a multiple of 8 bits, that is, only whole bytes.</p> + <p><em>Example:</em></p> <code type="none"><![CDATA[ <<Bin/binary,Bitstring/bitstring>>]]></code> <p>The variable <c>Bin</c> must contain a whole number of bytes, because the <c>binary</c> type defaults to <c>unit:8</c>. - A <c>badarg</c> exception will be generated if <c>Bin</c> would - consist of (for instance) 17 bits.</p> + A <c>badarg</c> exception is generated if <c>Bin</c> + consist of, for example, 17 bits.</p> - <p>On the other hand, the variable <c>Bitstring</c> may consist of - any number of bits, for instance 0, 1, 8, 11, 17, 42, and so on, - because the default <c>unit</c> for bitstrings is 1.</p> + <p>The <c>Bitstring</c> variable can consist of + any number of bits, for example, 0, 1, 8, 11, 17, 42, and so on. + This is because the default <c>unit</c> for bitstrings is 1.</p> - <warning><p>For clarity, it is recommended not to change the unit - size for binaries, but to use <c>binary</c> when you need byte - alignment, and <c>bitstring</c> when you need bit alignment.</p></warning> + <p>For clarity, it is recommended not to change the unit + size for binaries. Instead, use <c>binary</c> when you need byte alignment + and <c>bitstring</c> when you need bit alignment.</p> - <p>The following example</p> + <p>The following example successfully constructs a bitstring of 7 bits, + provided that all of X and Y are integers:</p> <code type="none"><![CDATA[ <<X:1,Y:6>>]]></code> - <p>will successfully construct a bitstring of 7 bits. - (Provided that all of X and Y are integers.)</p> - <p>As noted earlier, segments have the following general syntax:</p> + <p>As mentioned 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> + or a 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> + <p>This expression must be rewritten into the following, + to be accepted by the compiler:</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> + <p>A literal string can be written instead of an element:</p> <code type="none"><![CDATA[ <<"hello">>]]></code> - <p>which is syntactic sugar for</p> + <p>This is syntactic sugar for the following:</p> <code type="none"><![CDATA[ <<$h,$e,$l,$l,$o>>]]></code> </section> @@ -263,29 +271,30 @@ X:4/little-signed-integer-unit:8</code> <section> <title>Matching Binaries</title> - <p>This section describes the rules for matching binaries using + <marker id="Matching Binaries"></marker> + <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 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 (or divisible by the unit size, if the unit size has been changed).</p> - <p>A segment of type <c>bitstring</c> has no restrictions on the size.</p> - <p>As noted earlier, segments have the following general syntax:</p> + A binary pattern can occur wherever patterns are allowed, + including inside other patterns. Binary patterns cannot be nested. + The pattern <c><![CDATA[<<>>]]></c> matches a zero length binary.</p> + <p>Each segment in a binary can consist of zero or more bits. + A segment of type <c>binary</c> 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 <c>bitstring</c> has no restrictions on the size.</p> + <p>As mentioned 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 + <p>When matching <c>Value</c>, value must be either a variable or + an integer, or a 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> + variable. 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> + <p>The correct way to write this example is as follows:</p> <code type="none"><![CDATA[ foo(N, Bin) -> <<X:N,T/binary>> = Bin, @@ -303,14 +312,14 @@ foo(<<A:8,Rest/binary>>) ->]]></code> without size:</p> <code type="none"><![CDATA[ foo(<<A:8,Rest/bitstring>>) ->]]></code> - <p>There is no restriction on the number of bits in the tail.</p> + <p>There are no restrictions on the number of bits in the tail.</p> </section> </section> <section> <title>Appending to a Binary</title> - <p>In R12B, the following function for creating a binary out of - a list of triples of integers is now efficient:</p> + <p>Since Erlang R12B, the following function for creating a binary out of + a list of triples of integers is efficient:</p> <code type="none"><![CDATA[ triples_to_bin(T) -> triples_to_bin(T, <<>>). @@ -321,7 +330,8 @@ triples_to_bin([], Acc) -> Acc.]]></code> <p>In previous releases, this function was highly inefficient, because the binary constructed so far (<c>Acc</c>) was copied in each recursion step. - That is no longer the case. See the Efficiency Guide for more information.</p> + That is no longer the case. For more information, see + <seealso marker="doc/efficiency_guide">Efficiency Guide</seealso>.</p> </section> </chapter> |