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| <title>Using the GNU Compiler Collection (GCC): Disappointments</title> |
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| Next: <a href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" accesskey="n" rel="next">C++ Misunderstandings</a>, Previous: <a href="Standard-Libraries.html#Standard-Libraries" accesskey="p" rel="prev">Standard Libraries</a>, Up: <a href="Trouble.html#Trouble" accesskey="u" rel="up">Trouble</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Disappointments-and-Misunderstandings"></a> |
| <h3 class="section">11.6 Disappointments and Misunderstandings</h3> |
| |
| <p>These problems are perhaps regrettable, but we don’t know any practical |
| way around them. |
| </p> |
| <ul> |
| <li> Certain local variables aren’t recognized by debuggers when you compile |
| with optimization. |
| |
| <p>This occurs because sometimes GCC optimizes the variable out of |
| existence. There is no way to tell the debugger how to compute the |
| value such a variable “would have had”, and it is not clear that would |
| be desirable anyway. So GCC simply does not mention the eliminated |
| variable when it writes debugging information. |
| </p> |
| <p>You have to expect a certain amount of disagreement between the |
| executable and your source code, when you use optimization. |
| </p> |
| </li><li> <a name="index-conflicting-types"></a> |
| <a name="index-scope-of-declaration"></a> |
| Users often think it is a bug when GCC reports an error for code |
| like this: |
| |
| <div class="smallexample"> |
| <pre class="smallexample">int foo (struct mumble *); |
| |
| struct mumble { … }; |
| |
| int foo (struct mumble *x) |
| { … } |
| </pre></div> |
| |
| <p>This code really is erroneous, because the scope of <code>struct |
| mumble</code> in the prototype is limited to the argument list containing it. |
| It does not refer to the <code>struct mumble</code> defined with file scope |
| immediately below—they are two unrelated types with similar names in |
| different scopes. |
| </p> |
| <p>But in the definition of <code>foo</code>, the file-scope type is used |
| because that is available to be inherited. Thus, the definition and |
| the prototype do not match, and you get an error. |
| </p> |
| <p>This behavior may seem silly, but it’s what the ISO standard specifies. |
| It is easy enough for you to make your code work by moving the |
| definition of <code>struct mumble</code> above the prototype. It’s not worth |
| being incompatible with ISO C just to avoid an error for the example |
| shown above. |
| </p> |
| </li><li> Accesses to bit-fields even in volatile objects works by accessing larger |
| objects, such as a byte or a word. You cannot rely on what size of |
| object is accessed in order to read or write the bit-field; it may even |
| vary for a given bit-field according to the precise usage. |
| |
| <p>If you care about controlling the amount of memory that is accessed, use |
| volatile but do not use bit-fields. |
| </p> |
| </li><li> GCC comes with shell scripts to fix certain known problems in system |
| header files. They install corrected copies of various header files in |
| a special directory where only GCC will normally look for them. The |
| scripts adapt to various systems by searching all the system header |
| files for the problem cases that we know about. |
| |
| <p>If new system header files are installed, nothing automatically arranges |
| to update the corrected header files. They can be updated using the |
| <code>mkheaders</code> script installed in |
| <samp><var>libexecdir</var>/gcc/<var>target</var>/<var>version</var>/install-tools/</samp>. |
| </p> |
| </li><li> <a name="index-floating-point-precision"></a> |
| On 68000 and x86 systems, for instance, you can get paradoxical results |
| if you test the precise values of floating point numbers. For example, |
| you can find that a floating point value which is not a NaN is not equal |
| to itself. This results from the fact that the floating point registers |
| hold a few more bits of precision than fit in a <code>double</code> in memory. |
| Compiled code moves values between memory and floating point registers |
| at its convenience, and moving them into memory truncates them. |
| |
| <a name="index-ffloat_002dstore-1"></a> |
| <p>You can partially avoid this problem by using the <samp>-ffloat-store</samp> |
| option (see <a href="Optimize-Options.html#Optimize-Options">Optimize Options</a>). |
| </p> |
| </li><li> On AIX and other platforms without weak symbol support, templates |
| need to be instantiated explicitly and symbols for static members |
| of templates will not be generated. |
| |
| </li><li> On AIX, GCC scans object files and library archives for static |
| constructors and destructors when linking an application before the |
| linker prunes unreferenced symbols. This is necessary to prevent the |
| AIX linker from mistakenly assuming that static constructor or |
| destructor are unused and removing them before the scanning can occur. |
| All static constructors and destructors found will be referenced even |
| though the modules in which they occur may not be used by the program. |
| This may lead to both increased executable size and unexpected symbol |
| references. |
| </li></ul> |
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