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| <title>Debugging with GDB: Ada Glitches</title> |
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| <a name="Ada-Glitches"></a> |
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| Previous: <a href="Ravenscar-Profile.html#Ravenscar-Profile" accesskey="p" rel="prev">Ravenscar Profile</a>, Up: <a href="Ada.html#Ada" accesskey="u" rel="up">Ada</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Known-Peculiarities-of-Ada-Mode"></a> |
| <h4 class="subsubsection">15.4.9.9 Known Peculiarities of Ada Mode</h4> |
| <a name="index-Ada_002c-problems"></a> |
| |
| <p>Besides the omissions listed previously (see <a href="Omissions-from-Ada.html#Omissions-from-Ada">Omissions from Ada</a>), |
| we know of several problems with and limitations of Ada mode in |
| <small>GDB</small>, |
| some of which will be fixed with planned future releases of the debugger |
| and the GNU Ada compiler. |
| </p> |
| <ul> |
| <li> Static constants that the compiler chooses not to materialize as objects in |
| storage are invisible to the debugger. |
| |
| </li><li> Named parameter associations in function argument lists are ignored (the |
| argument lists are treated as positional). |
| |
| </li><li> Many useful library packages are currently invisible to the debugger. |
| |
| </li><li> Fixed-point arithmetic, conversions, input, and output is carried out using |
| floating-point arithmetic, and may give results that only approximate those on |
| the host machine. |
| |
| </li><li> The GNAT compiler never generates the prefix <code>Standard</code> for any of |
| the standard symbols defined by the Ada language. <small>GDB</small> knows about |
| this: it will strip the prefix from names when you use it, and will never |
| look for a name you have so qualified among local symbols, nor match against |
| symbols in other packages or subprograms. If you have |
| defined entities anywhere in your program other than parameters and |
| local variables whose simple names match names in <code>Standard</code>, |
| GNAT’s lack of qualification here can cause confusion. When this happens, |
| you can usually resolve the confusion |
| by qualifying the problematic names with package |
| <code>Standard</code> explicitly. |
| </li></ul> |
| |
| <p>Older versions of the compiler sometimes generate erroneous debugging |
| information, resulting in the debugger incorrectly printing the value |
| of affected entities. In some cases, the debugger is able to work |
| around an issue automatically. In other cases, the debugger is able |
| to work around the issue, but the work-around has to be specifically |
| enabled. |
| </p> |
| <a name="index-set-ada-trust_002dPAD_002dover_002dXVS"></a> |
| <a name="index-show-ada-trust_002dPAD_002dover_002dXVS"></a> |
| <dl compact="compact"> |
| <dt><code>set ada trust-PAD-over-XVS on</code></dt> |
| <dd><p>Configure GDB to strictly follow the GNAT encoding when computing the |
| value of Ada entities, particularly when <code>PAD</code> and <code>PAD___XVS</code> |
| types are involved (see <code>ada/exp_dbug.ads</code> in the GCC sources for |
| a complete description of the encoding used by the GNAT compiler). |
| This is the default. |
| </p> |
| </dd> |
| <dt><code>set ada trust-PAD-over-XVS off</code></dt> |
| <dd><p>This is related to the encoding using by the GNAT compiler. If <small>GDB</small> |
| sometimes prints the wrong value for certain entities, changing <code>ada |
| trust-PAD-over-XVS</code> to <code>off</code> activates a work-around which may fix |
| the issue. It is always safe to set <code>ada trust-PAD-over-XVS</code> to |
| <code>off</code>, but this incurs a slight performance penalty, so it is |
| recommended to leave this setting to <code>on</code> unless necessary. |
| </p> |
| </dd> |
| </dl> |
| |
| <a name="index-GNAT-descriptive-types"></a> |
| <a name="index-GNAT-encoding"></a> |
| <p>Internally, the debugger also relies on the compiler following a number |
| of conventions known as the ‘<samp>GNAT Encoding</samp>’, all documented in |
| <samp>gcc/ada/exp_dbug.ads</samp> in the GCC sources. This encoding describes |
| how the debugging information should be generated for certain types. |
| In particular, this convention makes use of <em>descriptive types</em>, |
| which are artificial types generated purely to help the debugger. |
| </p> |
| <p>These encodings were defined at a time when the debugging information |
| format used was not powerful enough to describe some of the more complex |
| types available in Ada. Since DWARF allows us to express nearly all |
| Ada features, the long-term goal is to slowly replace these descriptive |
| types by their pure DWARF equivalent. To facilitate that transition, |
| a new maintenance option is available to force the debugger to ignore |
| those descriptive types. It allows the user to quickly evaluate how |
| well <small>GDB</small> works without them. |
| </p> |
| <dl compact="compact"> |
| <dd> |
| <a name="index-maint-ada-set-ignore_002ddescriptive_002dtypes"></a> |
| </dd> |
| <dt><code>maintenance ada set ignore-descriptive-types [on|off]</code></dt> |
| <dd><p>Control whether the debugger should ignore descriptive types. |
| The default is not to ignore descriptives types (<code>off</code>). |
| </p> |
| <a name="index-maint-ada-show-ignore_002ddescriptive_002dtypes"></a> |
| </dd> |
| <dt><code>maintenance ada show ignore-descriptive-types</code></dt> |
| <dd><p>Show if descriptive types are ignored by <small>GDB</small>. |
| </p> |
| </dd> |
| </dl> |
| |
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