107 lines
5.3 KiB
HTML
107 lines
5.3 KiB
HTML
<html lang="en">
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<title>Structures - STABS</title>
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<link rel="up" href="Types.html#Types" title="Types">
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<link rel="prev" href="Enumerations.html#Enumerations" title="Enumerations">
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Copyright (C) 1992-2019 Free Software Foundation, Inc.
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Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon,
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and David MacKenzie.
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Permission is granted to copy, distribute and/or modify this document
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<a name="Structures"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Typedefs.html#Typedefs">Typedefs</a>,
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Previous: <a rel="previous" accesskey="p" href="Enumerations.html#Enumerations">Enumerations</a>,
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Up: <a rel="up" accesskey="u" href="Types.html#Types">Types</a>
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<hr>
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</div>
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<h3 class="section">5.8 Structures</h3>
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<p>The encoding of structures in stabs can be shown with an example.
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<p>The following source code declares a structure tag and defines an
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instance of the structure in global scope. Then a <code>typedef</code> equates the
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structure tag with a new type. Separate stabs are generated for the
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structure tag, the structure <code>typedef</code>, and the structure instance. The
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stabs for the tag and the <code>typedef</code> are emitted when the definitions are
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encountered. Since the structure elements are not initialized, the
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stab and code for the structure variable itself is located at the end
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of the program in the bss section.
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<pre class="example"> struct s_tag {
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int s_int;
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float s_float;
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char s_char_vec[8];
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struct s_tag* s_next;
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} g_an_s;
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typedef struct s_tag s_typedef;
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</pre>
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<p>The structure tag has an <code>N_LSYM</code> stab type because, like the
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enumeration, the symbol has file scope. Like the enumeration, the
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symbol descriptor is ‘<samp><span class="samp">T</span></samp>’, for enumeration, structure, or tag type.
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The type descriptor ‘<samp><span class="samp">s</span></samp>’ following the ‘<samp><span class="samp">16=</span></samp>’ of the type
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definition narrows the symbol type to structure.
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<p>Following the ‘<samp><span class="samp">s</span></samp>’ type descriptor is the number of bytes the
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structure occupies, followed by a description of each structure element.
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The structure element descriptions are of the form
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‘<samp><var>name</var><span class="samp">:</span><var>type</var><span class="samp">, </span><var>bit offset from the start of the
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struct</var><span class="samp">, </span><var>number of bits in the element</var></samp>’.
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<!-- FIXME: phony line break. Can probably be fixed by using an example -->
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<!-- with fewer fields. -->
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<pre class="example"> # <span class="roman">128 is N_LSYM</span>
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.stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
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s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
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</pre>
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<p>In this example, the first two structure elements are previously defined
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types. For these, the type following the ‘<samp><var>name</var><span class="samp">:</span></samp>’ part of the
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element description is a simple type reference. The other two structure
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elements are new types. In this case there is a type definition
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embedded after the ‘<samp><var>name</var><span class="samp">:</span></samp>’. The type definition for the
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array element looks just like a type definition for a stand-alone array.
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The <code>s_next</code> field is a pointer to the same kind of structure that
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the field is an element of. So the definition of structure type 16
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contains a type definition for an element which is a pointer to type 16.
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<p>If a field is a static member (this is a C<tt>++</tt> feature in which a single
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variable appears to be a field of every structure of a given type) it
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still starts out with the field name, a colon, and the type, but then
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instead of a comma, bit position, comma, and bit size, there is a colon
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followed by the name of the variable which each such field refers to.
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<p>If the structure has methods (a C<tt>++</tt> feature), they follow the non-method
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fields; see <a href="Cplusplus.html#Cplusplus">Cplusplus</a>.
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</body></html>
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