mingw/info/stabs/Stack-Variables.html

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  12 Node:<a name="Stack%20Variables">Stack Variables</a>,
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  17
  18 <h3 class="section">Automatic Variables Allocated on the Stack</h3>
  19
  20    <p>If a variable's scope is local to a function and its lifetime is only as
  21 long as that function executes (C calls such variables
  22 <dfn>automatic</dfn>), it can be allocated in a register (see <a href="Register-Variables.html#Register%20Variables">Register Variables</a>) or on the stack.
  23
  24    <p>Each variable allocated on the stack has a stab with the symbol
  25 descriptor omitted.  Since type information should begin with a digit,
  26 <code>-</code>, or <code>(</code>, only those characters precluded from being used
  27 for symbol descriptors.  However, the Acorn RISC machine (ARM) is said
  28 to get this wrong: it puts out a mere type definition here, without the
  29 preceding <code></code><var>type-number</var><code>=</code>.  This is a bad idea; there is no
  30 guarantee that type descriptors are distinct from symbol descriptors.
  31 Stabs for stack variables use the <code>N_LSYM</code> stab type, or
  32 <code>C_LSYM</code> for XCOFF.
  33
  34    <p>The value of the stab is the offset of the variable within the
  35 local variables.  On most machines this is an offset from the frame
  36 pointer and is negative.  The location of the stab specifies which block
  37 it is defined in; see <a href="Block-Structure.html#Block%20Structure">Block Structure</a>.
  38
  39    <p>For example, the following C code:
  40
  41 <pre class="example">     int
  42      main ()
  43      {
  44        int x;
  45      }
  46      </pre>
  47
  48    <p>produces the following stabs:
  49
  50 <pre class="example">     .stabs "main:F1",36,0,0,_main   # 36 is N_FUN
  51      .stabs "x:1",128,0,0,-12        # 128 is N_LSYM
  52      .stabn 192,0,0,LBB2             # 192 is N_LBRAC
  53      .stabn 224,0,0,LBE2             # 224 is N_RBRAC
  54      </pre>
  55
  56    <p>See <a href="Procedures.html#Procedures">Procedures</a> for more information on the <code>N_FUN</code> stab, and
  57 <a href="Block-Structure.html#Block%20Structure">Block Structure</a> for more information on the <code>N_LBRAC</code> and
  58 <code>N_RBRAC</code> stabs.
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