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| <title>The C Preprocessor: Character sets</title> |
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| <a name="Character-sets"></a> |
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| Next: <a href="Initial-processing.html#Initial-processing" accesskey="n" rel="next">Initial processing</a>, Up: <a href="Overview.html#Overview" accesskey="u" rel="up">Overview</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="Character-sets-1"></a> |
| <h3 class="section">1.1 Character sets</h3> |
| |
| <p>Source code character set processing in C and related languages is |
| rather complicated. The C standard discusses two character sets, but |
| there are really at least four. |
| </p> |
| <p>The files input to CPP might be in any character set at all. CPP’s |
| very first action, before it even looks for line boundaries, is to |
| convert the file into the character set it uses for internal |
| processing. That set is what the C standard calls the <em>source</em> |
| character set. It must be isomorphic with ISO 10646, also known as |
| Unicode. CPP uses the UTF-8 encoding of Unicode. |
| </p> |
| <p>The character sets of the input files are specified using the |
| <samp>-finput-charset=</samp> option. |
| </p> |
| <p>All preprocessing work (the subject of the rest of this manual) is |
| carried out in the source character set. If you request textual |
| output from the preprocessor with the <samp>-E</samp> option, it will be |
| in UTF-8. |
| </p> |
| <p>After preprocessing is complete, string and character constants are |
| converted again, into the <em>execution</em> character set. This |
| character set is under control of the user; the default is UTF-8, |
| matching the source character set. Wide string and character |
| constants have their own character set, which is not called out |
| specifically in the standard. Again, it is under control of the user. |
| The default is UTF-16 or UTF-32, whichever fits in the target’s |
| <code>wchar_t</code> type, in the target machine’s byte |
| order.<a name="DOCF1" href="#FOOT1"><sup>1</sup></a> Octal and hexadecimal escape sequences do not undergo |
| conversion; <tt>'\x12'</tt> has the value 0x12 regardless of the currently |
| selected execution character set. All other escapes are replaced by |
| the character in the source character set that they represent, then |
| converted to the execution character set, just like unescaped |
| characters. |
| </p> |
| <p>In identifiers, characters outside the ASCII range can only be |
| specified with the ‘<samp>\u</samp>’ and ‘<samp>\U</samp>’ escapes, not used |
| directly. If strict ISO C90 conformance is specified with an option |
| such as <samp>-std=c90</samp>, or <samp>-fno-extended-identifiers</samp> is |
| used, then those escapes are not permitted in identifiers. |
| </p> |
| <div class="footnote"> |
| <hr> |
| <h4 class="footnotes-heading">Footnotes</h4> |
| |
| <h3><a name="FOOT1" href="#DOCF1">(1)</a></h3> |
| <p>UTF-16 does not meet the requirements of the C |
| standard for a wide character set, but the choice of 16-bit |
| <code>wchar_t</code> is enshrined in some system ABIs so we cannot fix |
| this.</p> |
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| Next: <a href="Initial-processing.html#Initial-processing" accesskey="n" rel="next">Initial processing</a>, Up: <a href="Overview.html#Overview" accesskey="u" rel="up">Overview</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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