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133 lines
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<title>Ada Glitches - Debugging with GDB</title>
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<a name="Ada-Glitches"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Ada-Settings.html#Ada-Settings">Ada Settings</a>,
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Up: <a rel="up" accesskey="u" href="Ada.html#Ada">Ada</a>
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<h5 class="subsubsection">15.4.10.11 Known Peculiarities of Ada Mode</h5>
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<p><a name="index-Ada_002c-problems-1088"></a>
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Besides the omissions listed previously (see <a href="Omissions-from-Ada.html#Omissions-from-Ada">Omissions from Ada</a>),
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we know of several problems with and limitations of Ada mode in
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<span class="sc">gdb</span>,
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some of which will be fixed with planned future releases of the debugger
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and the GNU Ada compiler.
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<ul>
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<li>Static constants that the compiler chooses not to materialize as objects in
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storage are invisible to the debugger.
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<li>Named parameter associations in function argument lists are ignored (the
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argument lists are treated as positional).
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<li>Many useful library packages are currently invisible to the debugger.
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<li>Fixed-point arithmetic, conversions, input, and output is carried out using
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floating-point arithmetic, and may give results that only approximate those on
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the host machine.
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<li>The GNAT compiler never generates the prefix <code>Standard</code> for any of
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the standard symbols defined by the Ada language. <span class="sc">gdb</span> knows about
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this: it will strip the prefix from names when you use it, and will never
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look for a name you have so qualified among local symbols, nor match against
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symbols in other packages or subprograms. If you have
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defined entities anywhere in your program other than parameters and
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local variables whose simple names match names in <code>Standard</code>,
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GNAT's lack of qualification here can cause confusion. When this happens,
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you can usually resolve the confusion
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by qualifying the problematic names with package
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<code>Standard</code> explicitly.
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</ul>
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<p>Older versions of the compiler sometimes generate erroneous debugging
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information, resulting in the debugger incorrectly printing the value
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of affected entities. In some cases, the debugger is able to work
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around an issue automatically. In other cases, the debugger is able
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to work around the issue, but the work-around has to be specifically
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enabled.
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<p><a name="index-set-ada-trust_002dPAD_002dover_002dXVS-1089"></a><a name="index-show-ada-trust_002dPAD_002dover_002dXVS-1090"></a>
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<dl>
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<dt><code>set ada trust-PAD-over-XVS on</code><dd>Configure GDB to strictly follow the GNAT encoding when computing the
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value of Ada entities, particularly when <code>PAD</code> and <code>PAD___XVS</code>
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types are involved (see <code>ada/exp_dbug.ads</code> in the GCC sources for
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a complete description of the encoding used by the GNAT compiler).
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This is the default.
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<br><dt><code>set ada trust-PAD-over-XVS off</code><dd>This is related to the encoding using by the GNAT compiler. If <span class="sc">gdb</span>
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sometimes prints the wrong value for certain entities, changing <code>ada
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trust-PAD-over-XVS</code> to <code>off</code> activates a work-around which may fix
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the issue. It is always safe to set <code>ada trust-PAD-over-XVS</code> to
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<code>off</code>, but this incurs a slight performance penalty, so it is
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recommended to leave this setting to <code>on</code> unless necessary.
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</dl>
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<p><a name="index-GNAT-descriptive-types-1091"></a><a name="index-GNAT-encoding-1092"></a>Internally, the debugger also relies on the compiler following a number
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of conventions known as the ‘<samp><span class="samp">GNAT Encoding</span></samp>’, all documented in
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<samp><span class="file">gcc/ada/exp_dbug.ads</span></samp> in the GCC sources. This encoding describes
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how the debugging information should be generated for certain types.
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In particular, this convention makes use of <dfn>descriptive types</dfn>,
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which are artificial types generated purely to help the debugger.
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<p>These encodings were defined at a time when the debugging information
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format used was not powerful enough to describe some of the more complex
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types available in Ada. Since DWARF allows us to express nearly all
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Ada features, the long-term goal is to slowly replace these descriptive
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types by their pure DWARF equivalent. To facilitate that transition,
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a new maintenance option is available to force the debugger to ignore
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those descriptive types. It allows the user to quickly evaluate how
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well <span class="sc">gdb</span> works without them.
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<a name="index-maint-ada-set-ignore_002ddescriptive_002dtypes-1093"></a>
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<dl><dt><code>maintenance ada set ignore-descriptive-types [on|off]</code><dd>Control whether the debugger should ignore descriptive types.
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The default is not to ignore descriptives types (<code>off</code>).
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<p><a name="index-maint-ada-show-ignore_002ddescriptive_002dtypes-1094"></a><br><dt><code>maintenance ada show ignore-descriptive-types</code><dd>Show if descriptive types are ignored by <span class="sc">gdb</span>.
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</dl>
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