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| 22 | <title>Structures (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 33 | <link href="Referencing-Fields.html" rel="next" title="Referencing Fields">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Structures"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Arrays.html" accesskey="n" rel="next">Arrays</a>, Previous: <a href="Pointers.html" accesskey="p" rel="prev">Pointers</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Structures-1"></span><h2 class="chapter">15 Structures</h2>
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| 64 | <span id="index-structures"></span>
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| 65 | <span id="index-struct"></span>
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| 66 | <span id="index-fields-in-structures"></span>
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| 67 |
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| 68 | <p>A <em>structure</em> is a user-defined data type that holds various
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| 69 | <em>fields</em> of data. Each field has a name and a data type specified
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| 70 | in the structure’s definition.
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| 71 | </p>
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| 72 | <p>Here we define a structure suitable for storing a linked list of
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| 73 | integers. Each list item will hold one integer, plus a pointer
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| 74 | to the next item.
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| 75 | </p>
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| 76 | <div class="example">
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| 77 | <pre class="example">struct intlistlink
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| 78 | {
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| 79 | int datum;
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| 80 | struct intlistlink *next;
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| 81 | };
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| 82 | </pre></div>
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| 83 |
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| 84 | <p>The structure definition has a <em>type tag</em> so that the code can
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| 85 | refer to this structure. The type tag here is <code>intlistlink</code>.
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| 86 | The definition refers recursively to the same structure through that
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| 87 | tag.
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| 88 | </p>
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| 89 | <p>You can define a structure without a type tag, but then you can’t
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| 90 | refer to it again. That is useful only in some special contexts, such
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| 91 | as inside a <code>typedef</code> or a <code>union</code>.
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| 92 | </p>
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| 93 | <p>The contents of the structure are specified by the <em>field
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| 94 | declarations</em> inside the braces. Each field in the structure needs a
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| 95 | declaration there. The fields in one structure definition must have
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| 96 | distinct names, but these names do not conflict with any other names
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| 97 | in the program.
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| 98 | </p>
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| 99 | <p>A field declaration looks just like a variable declaration. You can
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| 100 | combine field declarations with the same beginning, just as you can
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| 101 | combine variable declarations.
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| 102 | </p>
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| 103 | <p>This structure has two fields. One, named <code>datum</code>, has type
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| 104 | <code>int</code> and will hold one integer in the list. The other, named
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| 105 | <code>next</code>, is a pointer to another <code>struct intlistlink</code>
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| 106 | which would be the rest of the list. In the last list item, it would
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| 107 | be <code>NULL</code>.
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| 108 | </p>
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| 109 | <p>This structure definition is recursive, since the type of the
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| 110 | <code>next</code> field refers to the structure type. Such recursion is not
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| 111 | a problem; in fact, you can use the type <code>struct intlistlink *</code>
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| 112 | before the definition of the type <code>struct intlistlink</code> itself.
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| 113 | That works because pointers to all kinds of structures really look the
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| 114 | same at the machine level.
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| 115 | </p>
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| 116 | <p>After defining the structure, you can declare a variable of type
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| 117 | <code>struct intlistlink</code> like this:
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| 118 | </p>
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| 119 | <div class="example">
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| 120 | <pre class="example">struct intlistlink foo;
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| 121 | </pre></div>
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| 122 |
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| 123 | <p>The structure definition itself can serve as the beginning of a
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| 124 | variable declaration, so you can declare variables immediately after,
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| 125 | like this:
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| 126 | </p>
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| 127 | <div class="example">
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| 128 | <pre class="example">struct intlistlink
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| 129 | {
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| 130 | int datum;
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| 131 | struct intlistlink *next;
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| 132 | } foo;
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| 133 | </pre></div>
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| 134 |
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| 135 | <p>But that is ugly. It is almost always clearer to separate the
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| 136 | definition of the structure from its uses.
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| 137 | </p>
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| 138 | <p>Declaring a structure type inside a block (see <a href="Blocks.html">Blocks</a>) limits
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| 139 | the scope of the structure type name to that block. That means the
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| 140 | structure type is recognized only within that block. Declaring it in
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| 141 | a function parameter list, as here,
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| 142 | </p>
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| 143 | <div class="example">
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| 144 | <pre class="example">int f (struct foo {int a, b} parm);
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| 145 | </pre></div>
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| 146 |
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| 147 | <p>(assuming that <code>struct foo</code> is not already defined) limits the
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| 148 | scope of the structure type <code>struct foo</code> to that parameter list;
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| 149 | that is basically useless, so it triggers a warning.
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| 150 | </p>
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| 151 | <p>Standard C requires at least one field in a structure.
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| 152 | GNU C does not require this.
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| 153 | </p>
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| 154 | <table class="menu" border="0" cellspacing="0">
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| 155 | <tr><td align="left" valign="top">• <a href="Referencing-Fields.html" accesskey="1">Referencing Fields</a></td><td> </td><td align="left" valign="top">Accessing field values in a structure object.
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| 156 | </td></tr>
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| 157 | <tr><td align="left" valign="top">• <a href="Dynamic-Memory-Allocation.html" accesskey="2">Dynamic Memory Allocation</a></td><td> </td><td align="left" valign="top">Allocating space for objects
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| 158 | while the program is running.
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| 159 | </td></tr>
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| 160 | <tr><td align="left" valign="top">• <a href="Field-Offset.html" accesskey="3">Field Offset</a></td><td> </td><td align="left" valign="top">Memory layout of fields within a structure.
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| 161 | </td></tr>
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| 162 | <tr><td align="left" valign="top">• <a href="Structure-Layout.html" accesskey="4">Structure Layout</a></td><td> </td><td align="left" valign="top">Planning the memory layout of fields.
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| 163 | </td></tr>
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| 164 | <tr><td align="left" valign="top">• <a href="Packed-Structures.html" accesskey="5">Packed Structures</a></td><td> </td><td align="left" valign="top">Packing structure fields as close as possible.
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| 165 | </td></tr>
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| 166 | <tr><td align="left" valign="top">• <a href="Bit-Fields.html" accesskey="6">Bit Fields</a></td><td> </td><td align="left" valign="top">Dividing integer fields
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| 167 | into fields with fewer bits.
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| 168 | </td></tr>
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| 169 | <tr><td align="left" valign="top">• <a href="Bit-Field-Packing.html" accesskey="7">Bit Field Packing</a></td><td> </td><td align="left" valign="top">How bit fields pack together in integers.
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| 170 | </td></tr>
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| 171 | <tr><td align="left" valign="top">• <a href="const-Fields.html" accesskey="8">const Fields</a></td><td> </td><td align="left" valign="top">Making structure fields immutable.
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| 172 | </td></tr>
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| 173 | <tr><td align="left" valign="top">• <a href="Zero-Length.html" accesskey="9">Zero Length</a></td><td> </td><td align="left" valign="top">Zero-length array as a variable-length object.
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| 174 | </td></tr>
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| 175 | <tr><td align="left" valign="top">• <a href="Flexible-Array-Fields.html">Flexible Array Fields</a></td><td> </td><td align="left" valign="top">Another approach to variable-length objects.
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| 176 | </td></tr>
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| 177 | <tr><td align="left" valign="top">• <a href="Overlaying-Structures.html">Overlaying Structures</a></td><td> </td><td align="left" valign="top">Casting one structure type
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| 178 | over an object of another structure type.
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| 179 | </td></tr>
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| 180 | <tr><td align="left" valign="top">• <a href="Structure-Assignment.html">Structure Assignment</a></td><td> </td><td align="left" valign="top">Assigning values to structure objects.
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| 181 | </td></tr>
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| 182 | <tr><td align="left" valign="top">• <a href="Unions.html">Unions</a></td><td> </td><td align="left" valign="top">Viewing the same object in different types.
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| 183 | </td></tr>
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| 184 | <tr><td align="left" valign="top">• <a href="Packing-With-Unions.html">Packing With Unions</a></td><td> </td><td align="left" valign="top">Using a union type to pack various types into
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| 185 | the same memory space.
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| 186 | </td></tr>
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| 187 | <tr><td align="left" valign="top">• <a href="Cast-to-Union.html">Cast to Union</a></td><td> </td><td align="left" valign="top">Casting a value one of the union’s alternative
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| 188 | types to the type of the union itself.
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| 189 | </td></tr>
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| 190 | <tr><td align="left" valign="top">• <a href="Structure-Constructors.html">Structure Constructors</a></td><td> </td><td align="left" valign="top">Building new structure objects.
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| 191 | </td></tr>
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| 192 | <tr><td align="left" valign="top">• <a href="Unnamed-Types-as-Fields.html">Unnamed Types as Fields</a></td><td> </td><td align="left" valign="top">Fields’ types do not always need names.
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| 193 | </td></tr>
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| 194 | <tr><td align="left" valign="top">• <a href="Incomplete-Types.html">Incomplete Types</a></td><td> </td><td align="left" valign="top">Types which have not been fully defined.
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| 195 | </td></tr>
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| 196 | <tr><td align="left" valign="top">• <a href="Intertwined-Incomplete-Types.html">Intertwined Incomplete Types</a></td><td> </td><td align="left" valign="top">Defining mutually-recursive structue types.
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| 197 | </td></tr>
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| 198 | <tr><td align="left" valign="top">• <a href="Type-Tags.html">Type Tags</a></td><td> </td><td align="left" valign="top">Scope of structure and union type tags.
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| 199 | </td></tr>
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| 200 | </table>
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| 201 |
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| 202 | <hr>
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| 203 | <div class="header">
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| 204 | <p>
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| 205 | Next: <a href="Arrays.html" accesskey="n" rel="next">Arrays</a>, Previous: <a href="Pointers.html" accesskey="p" rel="prev">Pointers</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 206 | </div>
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