86 lines
4.1 KiB
HTML
86 lines
4.1 KiB
HTML
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<title>Stack Variables - STABS</title>
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<a name="Stack-Variables"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Global-Variables.html#Global-Variables">Global Variables</a>,
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Up: <a rel="up" accesskey="u" href="Variables.html#Variables">Variables</a>
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<hr>
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</div>
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<h3 class="section">4.1 Automatic Variables Allocated on the Stack</h3>
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<p>If a variable's scope is local to a function and its lifetime is only as
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long as that function executes (C calls such variables
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<dfn>automatic</dfn>), it can be allocated in a register (see <a href="Register-Variables.html#Register-Variables">Register Variables</a>) or on the stack.
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<p><a name="index-N_005fLSYM_002c-for-stack-variables-25"></a><a name="index-C_005fLSYM-26"></a>Each variable allocated on the stack has a stab with the symbol
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descriptor omitted. Since type information should begin with a digit,
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‘<samp><span class="samp">-</span></samp>’, or ‘<samp><span class="samp">(</span></samp>’, only those characters precluded from being used
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for symbol descriptors. However, the Acorn RISC machine (ARM) is said
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to get this wrong: it puts out a mere type definition here, without the
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preceding ‘<samp><var>type-number</var><span class="samp">=</span></samp>’. This is a bad idea; there is no
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guarantee that type descriptors are distinct from symbol descriptors.
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Stabs for stack variables use the <code>N_LSYM</code> stab type, or
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<code>C_LSYM</code> for XCOFF.
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<p>The value of the stab is the offset of the variable within the
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local variables. On most machines this is an offset from the frame
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pointer and is negative. The location of the stab specifies which block
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it is defined in; see <a href="Block-Structure.html#Block-Structure">Block Structure</a>.
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<p>For example, the following C code:
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<pre class="example"> int
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main ()
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{
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int x;
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}
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</pre>
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<p>produces the following stabs:
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<pre class="example"> .stabs "main:F1",36,0,0,_main # <span class="roman">36 is N_FUN</span>
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.stabs "x:1",128,0,0,-12 # <span class="roman">128 is N_LSYM</span>
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.stabn 192,0,0,LBB2 # <span class="roman">192 is N_LBRAC</span>
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.stabn 224,0,0,LBE2 # <span class="roman">224 is N_RBRAC</span>
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</pre>
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<p>See <a href="Procedures.html#Procedures">Procedures</a> for more information on the <code>N_FUN</code> stab, and
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<a href="Block-Structure.html#Block-Structure">Block Structure</a> for more information on the <code>N_LBRAC</code> and
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<code>N_RBRAC</code> stabs.
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</body></html>
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