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182 lines
8.4 KiB
HTML
182 lines
8.4 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Disappointments (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Disappointments (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Disappointments (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Trouble.html#Trouble" rel="up" title="Trouble">
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<link href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" rel="next" title="C++ Misunderstandings">
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<link href="Standard-Libraries.html#Standard-Libraries" rel="prev" title="Standard Libraries">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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div.example {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Disappointments"></a>
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<div class="header">
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<p>
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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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</div>
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<hr>
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<a name="Disappointments-and-Misunderstandings"></a>
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<h3 class="section">13.6 Disappointments and Misunderstandings</h3>
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<p>These problems are perhaps regrettable, but we don’t know any practical
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way around them.
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</p>
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<ul>
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<li> Certain local variables aren’t recognized by debuggers when you compile
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with optimization.
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<p>This occurs because sometimes GCC optimizes the variable out of
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existence. There is no way to tell the debugger how to compute the
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value such a variable “would have had”, and it is not clear that would
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be desirable anyway. So GCC simply does not mention the eliminated
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variable when it writes debugging information.
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</p>
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<p>You have to expect a certain amount of disagreement between the
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executable and your source code, when you use optimization.
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</p>
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</li><li> <a name="index-conflicting-types"></a>
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<a name="index-scope-of-declaration"></a>
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Users often think it is a bug when GCC reports an error for code
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like this:
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<div class="smallexample">
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<pre class="smallexample">int foo (struct mumble *);
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struct mumble { … };
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int foo (struct mumble *x)
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{ … }
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</pre></div>
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<p>This code really is erroneous, because the scope of <code>struct
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mumble</code> in the prototype is limited to the argument list containing it.
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It does not refer to the <code>struct mumble</code> defined with file scope
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immediately below—they are two unrelated types with similar names in
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different scopes.
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</p>
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<p>But in the definition of <code>foo</code>, the file-scope type is used
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because that is available to be inherited. Thus, the definition and
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the prototype do not match, and you get an error.
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</p>
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<p>This behavior may seem silly, but it’s what the ISO standard specifies.
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It is easy enough for you to make your code work by moving the
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definition of <code>struct mumble</code> above the prototype. It’s not worth
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being incompatible with ISO C just to avoid an error for the example
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shown above.
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</p>
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</li><li> Accesses to bit-fields even in volatile objects works by accessing larger
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objects, such as a byte or a word. You cannot rely on what size of
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object is accessed in order to read or write the bit-field; it may even
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vary for a given bit-field according to the precise usage.
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<p>If you care about controlling the amount of memory that is accessed, use
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volatile but do not use bit-fields.
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</p>
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</li><li> GCC comes with shell scripts to fix certain known problems in system
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header files. They install corrected copies of various header files in
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a special directory where only GCC will normally look for them. The
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scripts adapt to various systems by searching all the system header
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files for the problem cases that we know about.
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<p>If new system header files are installed, nothing automatically arranges
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to update the corrected header files. They can be updated using the
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<code>mkheaders</code> script installed in
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<samp><var>libexecdir</var>/gcc/<var>target</var>/<var>version</var>/install-tools/</samp>.
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</p>
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</li><li> <a name="index-floating-point-precision"></a>
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On 68000 and x86 systems, for instance, you can get paradoxical results
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if you test the precise values of floating point numbers. For example,
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you can find that a floating point value which is not a NaN is not equal
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to itself. This results from the fact that the floating point registers
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hold a few more bits of precision than fit in a <code>double</code> in memory.
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Compiled code moves values between memory and floating point registers
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at its convenience, and moving them into memory truncates them.
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<a name="index-ffloat_002dstore-1"></a>
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<p>You can partially avoid this problem by using the <samp>-ffloat-store</samp>
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option (see <a href="Optimize-Options.html#Optimize-Options">Optimize Options</a>).
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</p>
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</li><li> On AIX and other platforms without weak symbol support, templates
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need to be instantiated explicitly and symbols for static members
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of templates will not be generated.
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</li><li> On AIX, GCC scans object files and library archives for static
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constructors and destructors when linking an application before the
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linker prunes unreferenced symbols. This is necessary to prevent the
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AIX linker from mistakenly assuming that static constructor or
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destructor are unused and removing them before the scanning can occur.
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All static constructors and destructors found will be referenced even
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though the modules in which they occur may not be used by the program.
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This may lead to both increased executable size and unexpected symbol
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references.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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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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</div>
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</body>
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</html>
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