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  17 <H2><A NAME="SEC65" HREF="suif1_toc.html#TOC65">Structure and Union Types</A></H2>
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  29 Structure and union types are defined with the <CODE>TYPE_STRUCT</CODE> and
  30 <CODE>TYPE_UNION</CODE> operators and use the <CODE>struct_type</CODE> derived type
  31 class.  Both of these types include a name and a list of fields.  The
  32 difference between them is that the fields of a union are all stored at
  33 the same offset so that only one field may be used at a time, while the
  34 fields of a structure are not allowed to overlap.  The <CODE>name</CODE>
  35 method returns the name of a structure or union type, which should be
  36 distinct from the names of other types within the symbol table where it
  37 is defined.  The name is automatically entered in the lexicon
  38 (see section <A HREF="suif1_102.html#SEC102">Lexicon</A>) when it is set with the <CODE>set_name</CODE> method.
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  43 The <CODE>struct_type</CODE> class contains a field to record the total size
  44 of the type in bits.  The <CODE>set_size</CODE> method is used to assign to
  45 this field.  Because structure and union types may be used as array
  46 elements, they must be able to tile an array without violating any
  47 alignment restrictions.  This means that extra padding may need to be
  48 added at the end of a structure or union.  Consequently, the total size
  49 of the type may be greater than the sizes of the fields.
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  61 The names, types, and offsets of the fields are stored in
  62 dynamically-allocated arrays within a <CODE>struct_type</CODE> node.  The
  63 <CODE>set_num_fields</CODE> method determines the number of fields and thus
  64 the size of these arrays.  The number of fields may be changed at any
  65 time.  If necessary, additional space will be allocated.  The
  66 <CODE>num_fields</CODE> method returns the current number of fields.  The
  67 <CODE>field_name</CODE>, <CODE>field_type</CODE>, and <CODE>offset</CODE> methods retrieve
  68 the field names, types, and offsets, and the <CODE>set_field_name</CODE>,
  69 <CODE>set_field_type</CODE>, and <CODE>set_offset</CODE> methods change their
  70 values.  The fields are numbered beginning with zero.  For structures
  71 the fields must be in order of increasing offsets.  The field offsets
  72 for union types should all be zero.  The field names are automatically
  73 entered in the lexicon (see section <A HREF="suif1_102.html#SEC102">Lexicon</A>).
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  79 The <CODE>find_field_by_name</CODE> method can be used to search for a field
  80 with a particular name.  If successful, it returns the index of the
  81 field; otherwise, it returns a value of <CODE>-1</CODE>.  The
  82 <CODE>find_field_by_offset</CODE> method can also be used for structure types
  83 to find the field at a certain offset.  If the specified offset is not
  84 exactly at the beginning of a field, <CODE>find_field_by_offset</CODE> returns
  85 the index of the field containing it and saves the offset within that
  86 field in the <CODE>left</CODE> parameter.
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