108 lines
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108 lines
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<title>Local Variable Parameters - STABS</title>
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<a name="Local-Variable-Parameters"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Reference-Parameters.html#Reference-Parameters">Reference Parameters</a>,
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Previous: <a rel="previous" accesskey="p" href="Register-Parameters.html#Register-Parameters">Register Parameters</a>,
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Up: <a rel="up" accesskey="u" href="Parameters.html#Parameters">Parameters</a>
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<h4 class="subsection">4.7.2 Storing Parameters as Local Variables</h4>
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<p>There is a case similar to an argument in a register, which is an
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argument that is actually stored as a local variable. Sometimes this
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happens when the argument was passed in a register and then the compiler
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stores it as a local variable. If possible, the compiler should claim
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that it's in a register, but this isn't always done.
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<p>If a parameter is passed as one type and converted to a smaller type by
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the prologue (for example, the parameter is declared as a <code>float</code>,
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but the calling conventions specify that it is passed as a
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<code>double</code>), then GCC2 (sometimes) uses a pair of symbols. The first
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symbol uses symbol descriptor ‘<samp><span class="samp">p</span></samp>’ and the type which is passed.
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The second symbol has the type and location which the parameter actually
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has after the prologue. For example, suppose the following C code
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appears with no prototypes involved:
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<pre class="example"> void
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subr (f)
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float f;
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{
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</pre>
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<p>if <code>f</code> is passed as a double at stack offset 8, and the prologue
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converts it to a float in register number 0, then the stabs look like:
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<pre class="example"> .stabs "f:p13",160,0,3,8 # <span class="roman">160 is </span><code>N_PSYM</code><span class="roman">, here 13 is </span><code>double</code>
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.stabs "f:r12",64,0,3,0 # <span class="roman">64 is </span><code>N_RSYM</code><span class="roman">, here 12 is </span><code>float</code>
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</pre>
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<p>In both stabs 3 is the line number where <code>f</code> is declared
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(see <a href="Line-Numbers.html#Line-Numbers">Line Numbers</a>).
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<p><a name="index-N_005fLSYM_002c-for-parameter-48"></a>GCC, at least on the 960, has another solution to the same problem. It
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uses a single ‘<samp><span class="samp">p</span></samp>’ symbol descriptor for an argument which is stored
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as a local variable but uses <code>N_LSYM</code> instead of <code>N_PSYM</code>. In
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this case, the value of the symbol is an offset relative to the local
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variables for that function, not relative to the arguments; on some
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machines those are the same thing, but not on all.
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<!-- This is mostly just background info; the part that logically belongs -->
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<!-- here is the last sentence. -->
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<p>On the VAX or on other machines in which the calling convention includes
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the number of words of arguments actually passed, the debugger (GDB at
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least) uses the parameter symbols to keep track of whether it needs to
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print nameless arguments in addition to the formal parameters which it
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has printed because each one has a stab. For example, in
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<pre class="example"> extern int fprintf (FILE *stream, char *format, ...);
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...
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fprintf (stdout, "%d\n", x);
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</pre>
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<p>there are stabs for <code>stream</code> and <code>format</code>. On most machines,
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the debugger can only print those two arguments (because it has no way
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of knowing that additional arguments were passed), but on the VAX or
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other machines with a calling convention which indicates the number of
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words of arguments, the debugger can print all three arguments. To do
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so, the parameter symbol (symbol descriptor ‘<samp><span class="samp">p</span></samp>’) (not necessarily
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‘<samp><span class="samp">r</span></samp>’ or symbol descriptor omitted symbols) needs to contain the
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actual type as passed (for example, <code>double</code> not <code>float</code> if it
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is passed as a double and converted to a float).
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</body></html>
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