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<p>Each individual type in the C <a href="compatible_type.html" title="c/language/type">type system</a> has several <i>qualified</i> versions of that type, corresponding to one, two, or all three of the <i>const</i>, <a href="volatile.html" title="c/language/volatile">volatile</a>, and, for pointers to object types, <a href="restrict.html" title="c/language/restrict">restrict</a> qualifiers. This page describes the effects of the <i>const</i> qualifier.
</p><p>Objects <a href="declarations.html" title="c/language/declarations">declared</a> with const-qualified types may be placed in read-only memory by the compiler, and if the address of a const object is never taken in a program, it may not be stored at all.
</p><p><code>const</code> semantics apply to <a href="value_category.html" title="c/language/value category">lvalue</a> expressions only; whenever a const lvalue expression is used in context that does not require an lvalue, its <code>const</code> qualifier is lost (note that volatile qualifier, if present, isn't lost).
</p><p>The lvalue expressions that designate objects of const-qualified type and the lvalue expressions that designate objects of struct or union type with at least one member of const-qualified type (including members of recursively contained aggregates or unions), are not <i>modifiable lvalues</i>. In particular, they are not assignable:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">const</span> <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// object of const type</span>
n <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// error: the type of n is const-qualified</span>
 
<span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// object of unqualified type</span>
<span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="sy3">&</span>x<span class="sy4">;</span>
<span class="sy2">*</span>p <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="co1">// error: the type of the lvalue *p is const-qualified</span>
 
<span class="kw1">struct</span> <span class="br0">{</span><span class="kw4">int</span> a<span class="sy4">;</span> <span class="kw4">const</span> <span class="kw4">int</span> b<span class="sy4">;</span> <span class="br0">}</span> s1 <span class="sy1">=</span> <span class="br0">{</span>.<span class="me1">b</span><span class="sy1">=</span><span class="nu0">1</span><span class="br0">}</span>, s2 <span class="sy1">=</span> <span class="br0">{</span>.<span class="me1">b</span><span class="sy1">=</span><span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
s1 <span class="sy1">=</span> s2<span class="sy4">;</span> <span class="co1">// error: the type of s1 is unqualified, but it has a const member</span></pre></div></div>
<p>A member of a const-qualified structure or union type acquires the qualification of the type it belongs to (both when accessed using the <code><b>.</b></code> operator or the <code><b>-></b></code> operator).
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw1">struct</span> s <span class="br0">{</span> <span class="kw4">int</span> i<span class="sy4">;</span> <span class="kw4">const</span> <span class="kw4">int</span> ci<span class="sy4">;</span> <span class="br0">}</span> s<span class="sy4">;</span>
<span class="co1">// the type of s.i is int, the type of s.ci is const int</span>
<span class="kw4">const</span> <span class="kw1">struct</span> s cs<span class="sy4">;</span>
<span class="co1">// the types of cs.i and cs.ci are both const int</span></pre></div></div>
<p>If an array type is declared with the const type qualifier (through the use of <a href="typedef.html" title="c/language/typedef">typedef</a>), the array type is not const-qualified, but its element type is. If a function type is declared with the const type qualifier (through the use of <a href="typedef.html" title="c/language/typedef">typedef</a>), the behavior is undefined.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw1">typedef</span> <span class="kw4">int</span> A<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span>
<span class="kw4">const</span> A a <span class="sy1">=</span> <span class="br0">{</span><span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">6</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">7</span>, <span class="nu0">8</span>, <span class="nu0">9</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// array of array of const int</span>
<span class="kw4">int</span><span class="sy2">*</span> pi <span class="sy1">=</span> a<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// Error: a[0] has type const int*</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-since-c99"><td>
<p>const-qualified compound literals do not necessarily designate distinct objects; they may share storage with other compound literals and with string literals that happen to have the same or overlapping representation.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span> p1 <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span><span class="br0">[</span><span class="br0">]</span><span class="br0">)</span><span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span>,<span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span> p2 <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span><span class="br0">[</span><span class="br0">]</span><span class="br0">)</span><span class="br0">{</span><span class="nu0">2</span>,<span class="nu0">3</span>,<span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// the value of p2 may equal p1+1</span>
bool b <span class="sy1">=</span> <span class="st0">"abd"</span> <span class="sy1">==</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="br0">[</span><span class="br0">]</span><span class="br0">)</span><span class="br0">{</span><span class="st0">"abc"</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// the value of b may be 1</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-c99">(since C99)</span></td></tr>
</table>
<p>A pointer to an non-const type can be implicitly converted to a pointer to const-qualified version of the same or <a href="compatible_type.html" title="c/language/compatible type" class="mw-redirect">compatible type</a>. The reverse conversion can be performed with a cast expression.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span> cp <span class="sy1">=</span> p<span class="sy4">;</span> <span class="co1">// OK: adds qualifiers (int to const int)</span>
p <span class="sy1">=</span> cp<span class="sy4">;</span> <span class="co1">// Error: discards qualifiers (const int to int)</span>
p <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span>cp<span class="sy4">;</span> <span class="co1">// OK: cast</span></pre></div></div>
<p>Note that pointer to pointer to <code>T</code> is not convertible to pointer to pointer to <code>const T</code>; for two types to be compatible, their qualifications must be identical.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">char</span> <span class="sy2">*</span>p <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">**</span>cpp <span class="sy1">=</span> <span class="sy3">&</span>p<span class="sy4">;</span> <span class="co1">// Error: char* and const char* are not compatible types</span>
<span class="kw4">char</span> <span class="sy2">*</span> <span class="kw4">const</span> <span class="sy2">*</span>pcp <span class="sy1">=</span> <span class="sy3">&</span>p<span class="sy4">;</span> <span class="co1">// OK, adds qualifiers (char* to char*const)</span></pre></div></div>
<p>Any attempt to modify an object whose type is const-qualified results in undefined behavior.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">const</span> <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// object of const-qualified type</span>
<span class="kw4">int</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><span class="sy3">&</span>n<span class="sy4">;</span>
<span class="sy2">*</span>p <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// undefined behavior</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-since-c99"><td>
<p>In a function declaration, the keyword <code>const</code> may appear inside the square brackets that are used to declare an array type of a function parameter. It qualifies the pointer type to which the array type is transformed.
</p><p>The following two declarations declare the same function:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">double</span> x<span class="br0">[</span><span class="kw4">const</span><span class="br0">]</span>, <span class="kw4">const</span> <span class="kw4">double</span> y<span class="br0">[</span><span class="kw4">const</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">double</span> <span class="sy2">*</span> <span class="kw4">const</span> x, <span class="kw4">const</span> <span class="kw4">double</span> <span class="sy2">*</span> <span class="kw4">const</span> y<span class="br0">)</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-c99">(since C99)</span></td></tr>
</table>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="const.html#Keywords"><span class="tocnumber">1</span> <span class="toctext">Keywords</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="const.html#Notes"><span class="tocnumber">2</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="const.html#References"><span class="tocnumber">3</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="const.html#See_Also"><span class="tocnumber">4</span> <span class="toctext">See Also</span></a></li>
</ul>
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/const&action=edit&section=1" title="Edit section: Keywords">edit</a>]</span> <span class="mw-headline" id="Keywords">Keywords</span></h3>
<p><a href="../../cpp/keyword/const.html" title="cpp/keyword/const"><tt>const</tt></a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/const&action=edit&section=2" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>C adopted the <i>const</i> qualifier from C++, but unlike in C++, expressions of const-qualified type in C are not <a href="constant_expression.html" title="c/language/constant expression">constant expressions</a>; they may not be used as <a href="switch.html" title="c/language/switch">case</a> labels or to initialize <a href="static_storage_duration.html" title="c/language/static storage duration">static</a> and <a href="thread_storage_duration.html" title="c/language/thread storage duration">thread</a> storage duration objects, <a href="enum.html" title="c/language/enum">enumerators</a>, or <a href="bit_field.html" title="c/language/bit field">bit field</a> sizes. When they are used as <a href="array.html" title="c/language/array">array</a> sizes, the resulting arrays are VLAs.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/const&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> 6.7.3 Type qualifiers (p: 121-123)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 6.7.3 Type qualifiers (p: 108-110)
</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> 3.5.3 Type qualifiers
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/const&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/cv.html" title="cpp/language/cv">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>cv (const and volatile) type qualifiers</span></span></div>
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