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| <title>The C Preprocessor: Implementation-defined behavior</title> |
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| <a name="Implementation_002ddefined-behavior"></a> |
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| Next: <a href="Implementation-limits.html#Implementation-limits" accesskey="n" rel="next">Implementation limits</a>, Up: <a href="Implementation-Details.html#Implementation-Details" accesskey="u" rel="up">Implementation Details</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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| <a name="Implementation_002ddefined-behavior-1"></a> |
| <h3 class="section">11.1 Implementation-defined behavior</h3> |
| <a name="index-implementation_002ddefined-behavior"></a> |
| |
| <p>This is how CPP behaves in all the cases which the C standard |
| describes as <em>implementation-defined</em>. This term means that the |
| implementation is free to do what it likes, but must document its choice |
| and stick to it. |
| </p> |
| <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>-finput-charset</samp> option, while the execution character set may |
| be controlled using the <samp>-fexec-charset</samp> and |
| <samp>-fwide-exec-charset</samp> options. |
| </p> |
| </li><li> Identifier characters. |
| <a name="Identifier-characters"></a> |
| <p>The C and C++ standards allow identifiers to be composed of ‘<samp>_</samp>’ |
| and the alphanumeric characters. C++ and C99 also allow universal |
| character names, and C99 further permits implementation-defined |
| characters. |
| </p> |
| <p>GCC allows the ‘<samp>$</samp>’ character in identifiers as an extension for |
| most targets. This is true regardless of the <samp>std=</samp> switch, |
| since this extension cannot conflict with standards-conforming |
| programs. When preprocessing assembler, however, dollars are not |
| identifier characters by default. |
| </p> |
| <p>Currently the targets that by default do not permit ‘<samp>$</samp>’ are AVR, |
| IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX |
| operating system. |
| </p> |
| <p>You can override the default with <samp>-fdollars-in-identifiers</samp> or |
| <samp>fno-dollars-in-identifiers</samp>. See <a href="Invocation.html#fdollars_002din_002didentifiers">fdollars-in-identifiers</a>. |
| </p> |
| </li><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. |
| </p> |
| </li><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>\a</samp>’ are given the |
| values they would have on the target machine. |
| </p> |
| <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> |
| <p>For example, <code>'ab'</code> for a target with an 8-bit <code>char</code> would be |
| interpreted as ‘<samp>(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')</samp>’<!-- /@w -->, and <code>'\234a'</code> as ‘<samp>(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')</samp>’<!-- /@w -->. |
| </p> |
| </li><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>. |
| </p> |
| </li><li> Interpretation of the filename resulting from a macro-expanded |
| ‘<samp>#include</samp>’ directive. |
| |
| <p>See <a href="Computed-Includes.html#Computed-Includes">Computed Includes</a>. |
| </p> |
| </li><li> Treatment of a ‘<samp>#pragma</samp>’ directive that after macro-expansion |
| results in a standard pragma. |
| |
| <p>No macro expansion occurs on any ‘<samp>#pragma</samp>’ directive line, so the |
| question does not arise. |
| </p> |
| <p>Note that GCC does not yet implement any of the standard |
| pragmas. |
| </p> |
| </li></ul> |
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| Next: <a href="Implementation-limits.html#Implementation-limits" accesskey="n" rel="next">Implementation limits</a>, Up: <a href="Implementation-Details.html#Implementation-Details" accesskey="u" rel="up">Implementation Details</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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