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| <h3 class="section">11.1 Implementation-defined behavior</h3> |
| |
| <p><a name="index-implementation_002ddefined-behavior-109"></a> |
| This is how CPP behaves in all the cases which the C standard |
| describes as <dfn>implementation-defined</dfn>. This term means that the |
| implementation is free to do what it likes, but must document its choice |
| and stick to it. |
| <!-- FIXME: Check the C++ standard for more implementation-defined stuff. --> |
| |
| <ul> |
| <li>The mapping of physical source file multi-byte characters to the |
| execution character set. |
| |
| <p>The input character set can be specified using the |
| <samp><span class="option">-finput-charset</span></samp> option, while the execution character set may |
| be controlled using the <samp><span class="option">-fexec-charset</span></samp> and |
| <samp><span class="option">-fwide-exec-charset</span></samp> options. |
| |
| <li>Identifier characters. |
| <a name="Identifier-characters"></a>The C and C++ standards allow identifiers to be composed of ‘<samp><span class="samp">_</span></samp>’ |
| and the alphanumeric characters. C++ and C99 also allow universal |
| character names, and C99 further permits implementation-defined |
| characters. GCC currently only permits universal character names if |
| <samp><span class="option">-fextended-identifiers</span></samp> is used, because the implementation of |
| universal character names in identifiers is experimental. |
| |
| <p>GCC allows the ‘<samp><span class="samp">$</span></samp>’ character in identifiers as an extension for |
| most targets. This is true regardless of the <samp><span class="option">std=</span></samp> switch, |
| since this extension cannot conflict with standards-conforming |
| programs. When preprocessing assembler, however, dollars are not |
| identifier characters by default. |
| |
| <p>Currently the targets that by default do not permit ‘<samp><span class="samp">$</span></samp>’ are AVR, |
| IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX |
| operating system. |
| |
| <p>You can override the default with <samp><span class="option">-fdollars-in-identifiers</span></samp> or |
| <samp><span class="option">fno-dollars-in-identifiers</span></samp>. See <a href="fdollars_002din_002didentifiers.html#fdollars_002din_002didentifiers">fdollars-in-identifiers</a>. |
| |
| <li>Non-empty sequences of whitespace characters. |
| |
| <p>In textual output, each whitespace sequence is collapsed to a single |
| space. For aesthetic reasons, the first token on each non-directive |
| line of output is preceded with sufficient spaces that it appears in the |
| same column as it did in the original source file. |
| |
| <li>The numeric value of character constants in preprocessor expressions. |
| |
| <p>The preprocessor and compiler interpret character constants in the |
| same way; i.e. escape sequences such as ‘<samp><span class="samp">\a</span></samp>’ are given the |
| values they would have on the target machine. |
| |
| <p>The compiler evaluates a multi-character character constant a character |
| at a time, shifting the previous value left by the number of bits per |
| target character, and then or-ing in the bit-pattern of the new |
| character truncated to the width of a target character. The final |
| bit-pattern is given type <code>int</code>, and is therefore signed, |
| regardless of whether single characters are signed or not (a slight |
| change from versions 3.1 and earlier of GCC). If there are more |
| characters in the constant than would fit in the target <code>int</code> the |
| compiler issues a warning, and the excess leading characters are |
| ignored. |
| |
| <p>For example, <code>'ab'</code> for a target with an 8-bit <code>char</code> would be |
| 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 -->. |
| |
| <li>Source file inclusion. |
| |
| <p>For a discussion on how the preprocessor locates header files, |
| <a href="Include-Operation.html#Include-Operation">Include Operation</a>. |
| |
| <li>Interpretation of the filename resulting from a macro-expanded |
| ‘<samp><span class="samp">#include</span></samp>’ directive. |
| |
| <p>See <a href="Computed-Includes.html#Computed-Includes">Computed Includes</a>. |
| |
| <li>Treatment of a ‘<samp><span class="samp">#pragma</span></samp>’ directive that after macro-expansion |
| results in a standard pragma. |
| |
| <p>No macro expansion occurs on any ‘<samp><span class="samp">#pragma</span></samp>’ directive line, so the |
| question does not arise. |
| |
| <p>Note that GCC does not yet implement any of the standard |
| pragmas. |
| |
| </ul> |
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