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