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143 lines
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<a name="Implementation-defined-behavior"></a>
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<a name="Implementation_002ddefined-behavior"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Implementation-limits.html#Implementation-limits">Implementation limits</a>,
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Up: <a rel="up" accesskey="u" href="Implementation-Details.html#Implementation-Details">Implementation Details</a>
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<h3 class="section">11.1 Implementation-defined behavior</h3>
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<p><a name="index-implementation_002ddefined-behavior-111"></a>
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This is how CPP behaves in all the cases which the C standard
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describes as <dfn>implementation-defined</dfn>. This term means that the
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implementation is free to do what it likes, but must document its choice
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and stick to it.
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<!-- FIXME: Check the C++ standard for more implementation-defined stuff. -->
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<ul>
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<li>The mapping of physical source file multi-byte characters to the
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execution character set.
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<p>The input character set can be specified using the
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<samp><span class="option">-finput-charset</span></samp> option, while the execution character set may
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be controlled using the <samp><span class="option">-fexec-charset</span></samp> and
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<samp><span class="option">-fwide-exec-charset</span></samp> options.
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<li>Identifier characters.
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<a name="Identifier-characters"></a>The C and C++ standards allow identifiers to be composed of ‘<samp><span class="samp">_</span></samp>’
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and the alphanumeric characters. C++ and C99 also allow universal
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character names, and C99 further permits implementation-defined
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characters.
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<p>GCC allows the ‘<samp><span class="samp">$</span></samp>’ character in identifiers as an extension for
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most targets. This is true regardless of the <samp><span class="option">std=</span></samp> switch,
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since this extension cannot conflict with standards-conforming
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programs. When preprocessing assembler, however, dollars are not
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identifier characters by default.
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<p>Currently the targets that by default do not permit ‘<samp><span class="samp">$</span></samp>’ are AVR,
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IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX
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operating system.
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<p>You can override the default with <samp><span class="option">-fdollars-in-identifiers</span></samp> or
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<samp><span class="option">fno-dollars-in-identifiers</span></samp>. See <a href="fdollars_002din_002didentifiers.html#fdollars_002din_002didentifiers">fdollars-in-identifiers</a>.
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<li>Non-empty sequences of whitespace characters.
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<p>In textual output, each whitespace sequence is collapsed to a single
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space. For aesthetic reasons, the first token on each non-directive
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line of output is preceded with sufficient spaces that it appears in the
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same column as it did in the original source file.
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<li>The numeric value of character constants in preprocessor expressions.
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<p>The preprocessor and compiler interpret character constants in the
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same way; i.e. escape sequences such as ‘<samp><span class="samp">\a</span></samp>’ are given the
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values they would have on the target machine.
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<p>The compiler evaluates a multi-character character constant a character
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at a time, shifting the previous value left by the number of bits per
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target character, and then or-ing in the bit-pattern of the new
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character truncated to the width of a target character. The final
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bit-pattern is given type <code>int</code>, and is therefore signed,
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regardless of whether single characters are signed or not (a slight
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change from versions 3.1 and earlier of GCC). If there are more
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characters in the constant than would fit in the target <code>int</code> the
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compiler issues a warning, and the excess leading characters are
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ignored.
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<p>For example, <code>'ab'</code> for a target with an 8-bit <code>char</code> would be
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interpreted as ‘<samp><span class="samp">(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')</span></samp>’<!-- /@w -->, and <code>'\234a'</code> as ‘<samp><span class="samp">(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')</span></samp>’<!-- /@w -->.
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<li>Source file inclusion.
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<p>For a discussion on how the preprocessor locates header files,
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<a href="Include-Operation.html#Include-Operation">Include Operation</a>.
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<li>Interpretation of the filename resulting from a macro-expanded
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‘<samp><span class="samp">#include</span></samp>’ directive.
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<p>See <a href="Computed-Includes.html#Computed-Includes">Computed Includes</a>.
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<li>Treatment of a ‘<samp><span class="samp">#pragma</span></samp>’ directive that after macro-expansion
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results in a standard pragma.
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<p>No macro expansion occurs on any ‘<samp><span class="samp">#pragma</span></samp>’ directive line, so the
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question does not arise.
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<p>Note that GCC does not yet implement any of the standard
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pragmas.
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</ul>
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