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<p>An <i>identifier</i> is an arbitrarily long sequence of digits, underscores, lowercase and uppercase Latin letters<span class="t-rev-inl t-since-c99"><span>, and Unicode characters specified using <code>\u</code> and <code>\U</code> <a href="escape.html" title="c/language/escape">escape</a> notation</span> <span><span class="t-mark-rev t-since-c99">(since C99)</span></span></span>. A valid identifier must begin with a non-digit character (Latin letter, underscore<span class="t-rev-inl t-since-c99"><span>, or Unicode non-digit character</span> <span><span class="t-mark-rev t-since-c99">(since C99)</span></span></span>). Identifiers are case-sensitive (lowercase and uppercase letters are distinct).<span class="t-rev-inl t-since-c99"><span> It is implementation-defined if raw (not escaped) Unicode characters are allowed in identifiers.</span> <span><span class="t-mark-rev t-since-c99">(since C99)</span></span></span>
</p><p>Identifiers can denote the following types of entities:
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</li><li> structure or union members
</li><li> enumeration constants
</li><li> <a href="typedef.html" title="c/language/typedef">typedef</a> names
</li><li> <a href="statements.html#Labels" title="c/language/statements">label</a> names
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</li><li> <a href="../preprocessor/replace.html" title="c/preprocessor/replace">macro parameter</a> names
</li></ul>
<p>Every identifier other than macro name or macro parameter name has <a href="scope.html" title="c/language/scope">scope</a>, belongs to a <a href="name_space.html" title="c/language/name space">name space</a>, and may have <a href="storage_duration.html" title="c/language/storage duration">linkage</a>. The same identifier can denote different entities at different points in the program, or may denote different entities at the same point if the entities are in different name spaces.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="identifier.html#Reserved_identifiers"><span class="tocnumber">1</span> <span class="toctext">Reserved identifiers</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="identifier.html#Translation_limits"><span class="tocnumber">2</span> <span class="toctext">Translation limits</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="identifier.html#References"><span class="tocnumber">3</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="identifier.html#See_Also"><span class="tocnumber">4</span> <span class="toctext">See Also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/identifier&action=edit&section=1" title="Edit section: Reserved identifiers">edit</a>]</span> <span class="mw-headline" id="Reserved_identifiers">Reserved identifiers</span></h3>
<p>The following identifiers are <i>reserved</i> and may not be declared in a program (doing so invokes undefined behavior):
</p>
<div class="t-li1"><span class="t-li">1)</span> The identifiers that are <a href="../keyword.html" title="c/keyword">keywords</a> cannot be used for other purposes. In particular #define or #undef of an identifier that is identical to a keyword is not allowed.</div>
<div class="t-li1"><span class="t-li">2)</span> All external identifiers that begin with an underscore.</div>
<div class="t-li1"><span class="t-li">3)</span> All identifiers that begin with an underscore followed by a capital letter or by another underscore (these reserved identifiers allow the library to use numerous behind-the-scenes non-external macros and functions)</div>
<div class="t-li1"><span class="t-li">4)</span> All external identifiers defined by the standard library (in hosted environment). This means that no user-supplied external names are allowed to match any library names, not even if declaring a function that is identical to a library function.</div>
<div class="t-li1"><span class="t-li">5)</span> Identifiers declared as reserved for future use by the standard library, namely
<dl><dd><ul><li> <i>function names</i>
</li></ul>
<dl><dd><ul><li> <code>cerf</code>, <code>cerfc</code>, <code>cexp2</code>, <code>cexpm1</code>, <code>clog10</code>, <code>clog1p</code>, <code>clog2</code>, <code>clgamma</code>, <code>ctgamma</code> and their -f and -l suffixed variants, in <a href="../numeric/complex.html" title="c/numeric/complex"><tt><complex.h></tt></a>
</li><li> beginning with <code>is</code> or <code>to</code> followed by a lowercase letter, in <a href="../string/byte.html" title="c/string/byte"><tt><ctype.h></tt></a> and <a href="../string/wide.html" title="c/string/wide"><tt>wctype.h</tt></a>
</li><li> beginning with <code>str</code> followed by a lowercase letter, in <a href="../string/byte.html" title="c/string/byte"><tt><stdlib.h></tt></a>
</li><li> beginning with <code>str</code>, <code>mem</code> or <code>wcs</code> followed by a lowercase letter, in <a href="../string/byte.html" title="c/string/byte"><tt><string.h></tt></a>
</li><li> beginning with <code>wcs</code> followed by a lowercase letter, in <a href="../string/wide.html" title="c/string/wide"><tt><wchar.h></tt></a>
</li><li> beginning with <code>atomic_</code> followed by a lowercase letter, in <a href="../atomic.html" title="c/atomic"><tt><stdatomic.h></tt></a>
</li><li> beginning with <code>cnd_</code>, <code>mtx_</code>, <code>thrd_</code> or <code>tss_</code> followed by a lowercase letter, in <a href="../thread.html" title="c/thread"><tt><threads.h></tt></a>
</li></ul>
</dd></dl>
<ul><li> <i>typedef names</i>
</li></ul>
<dl><dd><ul><li> beginning with <code>int</code> or <code>uint</code> and ending with <code>_t</code>, in <a href="../types/integer.html" title="c/types/integer"><tt><stdint.h></tt></a>
</li><li> beginning with <code>atomic_</code> or <code>memory_</code> followed by a lowercase letter, in <a href="../atomic.html" title="c/atomic"><tt><stdatomic.h></tt></a>
</li><li> beginning with <code>cnd_</code>, <code>mtx_</code>, <code>thrd_</code> or <code>tss_</code> followed by a lowercase letter, in <a href="../thread.html" title="c/thread"><tt><threads.h></tt></a>
</li></ul>
</dd></dl>
<ul><li> <i>macro names</i>
</li></ul>
<dl><dd><ul><li> beginning with <code>E</code> followed by a digit or an uppercase letter, in <a href="../error/errno_macros.html" title="c/error/errno macros"><tt><errno.h></tt></a>
</li><li> beginning with <code>FE_</code> followed by an uppercase letter, in <a href="../numeric/fenv.html" title="c/numeric/fenv"><tt><fenv.h></tt></a>
</li><li> beginning with <code>INT</code> or <code>UINT</code> and ending with <code>_MAX</code>, <code>_MIN</code>, or <code>_C</code>, in <a href="../types/integer.html" title="c/types/integer"><tt><stdint.h></tt></a>
</li><li> beginning with <code>PRI</code> or <code>SCN</code> followed by lowercase letter or the letter <code>X</code>, in <a href="../types/integer.html" title="c/types/integer"><tt><stdint.h></tt></a>
</li><li> beginning with <code>LC_</code> followed by an uppercase letter, in <a href="../locale/LC_categories.html" title="c/locale/LC categories"><tt><locale.h></tt></a>
</li><li> beginning with <code>SIG</code> or <code>SIG_</code> followed by an uppercase letter, in <a href="../program.html" title="c/program"><tt><signal.h></tt></a>
</li><li> beginning with <code>TIME_</code> followed by an uppercase letter, in <a href="../chrono/timespec_get.html" title="c/chrono/timespec get"><time.h></a>
</li><li> beginning with <code>ATOMIC_</code> followed by an uppercase letter, in <a href="../atomic.html" title="c/atomic"><tt><stdatomic.h></tt></a>
</li></ul>
</dd></dl>
<ul><li> <i>enumeration constants</i>
</li></ul>
<dl><dd><ul><li> beginning with <code>memory_order_</code> followed by a lowercase letter, in <a href="../atomic.html" title="c/atomic"><tt><stdatomic.h></tt></a>
</li><li> beginning with <code>cnd_</code>, <code>mtx_</code>, <code>thrd_</code> or <code>tss_</code> followed by a lowercase letter, in <a href="../thread.html" title="c/thread"><tt><threads.h></tt></a></div>
</li></ul>
</dd></dl>
</dd></dl>
<p>All other identifiers are available, with no fear of unexpected collisions when moving programs from one compiler and library to another.
</p><p>Note: in C++, identifiers with a double underscore anywhere are reserved everywhere; in C, only the ones that begin with a double underscore are reserved.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/identifier&action=edit&section=2" title="Edit section: Translation limits">edit</a>]</span> <span class="mw-headline" id="Translation_limits">Translation limits</span></h3>
<p>Even though there is no specific limit on the length of identifiers, early compilers had limits on the number of significant initial characters in identifiers and the linkers imposed stricter limits on the names with <a href="storage_duration.html" title="c/language/storage duration">external linkage</a>. C requires that at least the following limits are supported by any standard-compliant implementation:
</p>
<table class="t-rev-begin">
<tr class="t-rev t-until-c99"><td>
<ul><li>31 significant initial characters in an internal identifier or a macro name
</li><li>6 significant initial characters in an external identifier
</li><li>511 external identifiers in one translation unit
</li><li>127 identifiers with block scope declared in one block
</li><li>1024 macro identifiers simultaneously defined in one preprocessing translation unit
</li></ul>
</td>
<td><span class="t-mark">(until C99)</span></td></tr>
<tr class="t-rev t-since-c99"><td>
<ul><li>63 significant initial characters in an internal identifier or a macro name
</li><li>31 significant initial characters in an external identifier
</li><li>4095 external identifiers in one translation unit
</li><li>511 identifiers with block scope declared in one block
</li><li>4095 macro identifiers simultaneously defined in one preprocessing translation unit
</li></ul>
</td>
<td><span class="t-mark-rev t-since-c99">(since C99)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/identifier&action=edit&section=3" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011):
</li></ul>
<dl><dd><ul><li> 5.2.4.1 Translation limits (p: 25-26)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.4.2 Identifiers (p: 59-60)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.10.8 Predefined macro names (p: 175-176)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.11.9 Predefined macro names (p: 179)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 7.31 Future library directions (p: 455-457)
</li></ul>
</dd></dl>
<dl><dd><ul><li> K.3.1.2 Reserved identifiers (p: 584)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 5.2.4.1 Translation limits (p: 20-21)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.4.2 Identifiers (p: 51-52)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.10.8 Predefined macro names (p: 160-161)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.11.9 Predefined macro names (p: 163)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 7.26 Future library directions (p: 401-402)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990):
</li></ul>
<dl><dd><ul><li> 2.2.4.1 Translation limits
</li></ul>
</dd></dl>
<dl><dd><ul><li> 3.1.2 Identifiers
</li></ul>
</dd></dl>
<dl><dd><ul><li> 3.8.8 Predefined macro names
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/identifier&action=edit&section=4" title="Edit section: See Also">edit</a>]</span> <span class="mw-headline" id="See_Also">See Also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../cpp/language/name.html" title="cpp/language/identifiers">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>Identifiers</span></span></div>
</td></tr>
</table></div>
</div>
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