129 lines
6.3 KiB
HTML
129 lines
6.3 KiB
HTML
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<title>Arrays - STABS</title>
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<link rel="up" href="Types.html#Types" title="Types">
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<link rel="prev" href="Subranges.html#Subranges" title="Subranges">
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Copyright (C) 1992-2019 Free Software Foundation, Inc.
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<a name="Arrays"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Strings.html#Strings">Strings</a>,
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Previous: <a rel="previous" accesskey="p" href="Subranges.html#Subranges">Subranges</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">5.5 Array Types</h3>
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<p>Arrays use the ‘<samp><span class="samp">a</span></samp>’ type descriptor. Following the type descriptor
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is the type of the index and the type of the array elements. If the
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index type is a range type, it ends in a semicolon; otherwise
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(for example, if it is a type reference), there does not
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appear to be any way to tell where the types are separated. In an
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effort to clean up this mess, IBM documents the two types as being
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separated by a semicolon, and a range type as not ending in a semicolon
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(but this is not right for range types which are not array indexes,
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see <a href="Subranges.html#Subranges">Subranges</a>). I think probably the best solution is to specify
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that a semicolon ends a range type, and that the index type and element
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type of an array are separated by a semicolon, but that if the index
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type is a range type, the extra semicolon can be omitted. GDB (at least
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through version 4.9) doesn't support any kind of index type other than a
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range anyway; I'm not sure about dbx.
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<p>It is well established, and widely used, that the type of the index,
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unlike most types found in the stabs, is merely a type definition, not
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type information (see <a href="String-Field.html#String-Field">String Field</a>) (that is, it need not start with
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‘<samp><var>type-number</var><span class="samp">=</span></samp>’ if it is defining a new type). According to a
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comment in GDB, this is also true of the type of the array elements; it
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gives ‘<samp><span class="samp">ar1;1;10;ar1;1;10;4</span></samp>’ as a legitimate way to express a two
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dimensional array. According to AIX documentation, the element type
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must be type information. GDB accepts either.
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<p>The type of the index is often a range type, expressed as the type
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descriptor ‘<samp><span class="samp">r</span></samp>’ and some parameters. It defines the size of the
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array. In the example below, the range ‘<samp><span class="samp">r1;0;2;</span></samp>’ defines an index
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type which is a subrange of type 1 (integer), with a lower bound of 0
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and an upper bound of 2. This defines the valid range of subscripts of
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a three-element C array.
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<p>For example, the definition:
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<pre class="example"> char char_vec[3] = {'a','b','c'};
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</pre>
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<p class="noindent">produces the output:
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<pre class="example"> .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
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.global _char_vec
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.align 4
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_char_vec:
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.byte 97
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.byte 98
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.byte 99
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</pre>
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<p>If an array is <dfn>packed</dfn>, the elements are spaced more
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closely than normal, saving memory at the expense of speed. For
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example, an array of 3-byte objects might, if unpacked, have each
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element aligned on a 4-byte boundary, but if packed, have no padding.
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One way to specify that something is packed is with type attributes
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(see <a href="String-Field.html#String-Field">String Field</a>). In the case of arrays, another is to use the
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‘<samp><span class="samp">P</span></samp>’ type descriptor instead of ‘<samp><span class="samp">a</span></samp>’. Other than specifying a
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packed array, ‘<samp><span class="samp">P</span></samp>’ is identical to ‘<samp><span class="samp">a</span></samp>’.
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<!-- FIXME-what is it? A pointer? -->
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<p>An open array is represented by the ‘<samp><span class="samp">A</span></samp>’ type descriptor followed by
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type information specifying the type of the array elements.
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<!-- FIXME: what is the format of this type? A pointer to a vector of pointers? -->
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<p>An N-dimensional dynamic array is represented by
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<pre class="example"> D <var>dimensions</var> ; <var>type-information</var>
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</pre>
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<!-- Does dimensions really have this meaning? The AIX documentation -->
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<!-- doesn't say. -->
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<p><var>dimensions</var> is the number of dimensions; <var>type-information</var>
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specifies the type of the array elements.
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<!-- FIXME: what is the format of this type? A pointer to some offsets in -->
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<!-- another array? -->
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<p>A subarray of an N-dimensional array is represented by
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<pre class="example"> E <var>dimensions</var> ; <var>type-information</var>
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</pre>
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<!-- Does dimensions really have this meaning? The AIX documentation -->
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<!-- doesn't say. -->
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<p><var>dimensions</var> is the number of dimensions; <var>type-information</var>
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specifies the type of the array elements.
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</body></html>
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