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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="history.html" title="c/language/history"><span class="t-lines"><span>history of C</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> <a href="../experimental.html" title="c/experimental">Technical Specifications</a> </td></tr>
</table></div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:c/language/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>The C language standard precisely specifies the <a href="http://en.cppreference.com/mwiki/index.php?title=c/language/as_if&action=edit&redlink=1" class="new" title="c/language/as if (page does not exist)">observable behavior</a> of C language programs, except for the ones in the following categories:
</p>
<ul><li> <i>undefined behavior</i> - there are no restrictions on the behavior of the program. Examples of undefined behavior are memory accesses outside of array bounds, signed integer overflow, null pointer dereference, modification of the same scalar <a href="eval_order.html" title="c/language/eval order">more than once</a> in an expression without sequence points, access to an object through a pointer of a different type, etc. Compilers are not required to diagnose undefined behavior (although many simple situations are diagnosed), and the compiled program is not required to do anything meaningful.
</li></ul>
<ul><li> <i>unspecified behavior</i> - two or more behaviors are permitted and the implementation is not required to document the effects of each behavior. For example, <a href="eval_order.html" title="c/language/eval order">order of evaluation</a>, whether identical <a href="string_literal.html" title="c/language/string literal">string literals</a> are distinct, etc. Each unspecified behavior results in one of a set of valid results and may produce a different result when repeated in the same program.
</li></ul>
<ul><li> <i>implementation-defined behavior</i> - unspecified behavior where each implementation documents how the choice is made. For example, number of bits in a byte, or whether signed integer right shift is arithmetic or logical.
</li></ul>
<ul><li> <i>locale-specific behavior</i> - implementation-defined behavior that depends on the <a href="../locale/setlocale.html" title="c/locale/setlocale">currently chosen locale</a>. For example, whether <span class="t-lc"><a href="../string/byte/islower.html" title="c/string/byte/islower">islower</a></span> returns true for any character other than the 26 lowercase Latin letters.
</li></ul>
<p>(Note: <a href="conformance.html" title="c/language/conformance">Strictly conforming</a> programs do not depend on any unspecified, undefined, or implementation-defined behavior)
</p><p>The compilers are required to issue diagnostic messages (either errors or warnings) for any programs that violates any C syntax rule or semantic constraint, even if its behavior is specified as undefined or implementation-defined or if the compiler provides a language extension that allows it to accept such program. Diagnostics for undefined behavior are not otherwise required.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="behavior.html#UB_and_optimization"><span class="tocnumber">1</span> <span class="toctext">UB and optimization</span></a>
<ul>
<li class="toclevel-2 tocsection-2"><a href="behavior.html#Signed_overflow"><span class="tocnumber">1.1</span> <span class="toctext">Signed overflow</span></a></li>
<li class="toclevel-2 tocsection-3"><a href="behavior.html#Access_out_of_bounds"><span class="tocnumber">1.2</span> <span class="toctext">Access out of bounds</span></a></li>
<li class="toclevel-2 tocsection-4"><a href="behavior.html#Uninitialized_scalar"><span class="tocnumber">1.3</span> <span class="toctext">Uninitialized scalar</span></a></li>
<li class="toclevel-2 tocsection-5"><a href="behavior.html#Access_to_pointer_passed_to_realloc"><span class="tocnumber">1.4</span> <span class="toctext">Access to pointer passed to realloc</span></a></li>
<li class="toclevel-2 tocsection-6"><a href="behavior.html#Infinite_loop_without_side-effects"><span class="tocnumber">1.5</span> <span class="toctext">Infinite loop without side-effects</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-7"><a href="behavior.html#References"><span class="tocnumber">2</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-8"><a href="behavior.html#External_links"><span class="tocnumber">3</span> <span class="toctext">External links</span></a></li>
<li class="toclevel-1 tocsection-9"><a href="behavior.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/behavior&action=edit&section=1" title="Edit section: UB and optimization">edit</a>]</span> <span class="mw-headline" id="UB_and_optimization">UB and optimization</span></h3>
<p>Because correct C programs are free of undefined behavior, compilers may produce unexpected results when a program that actually has UB is compiled with optimization enabled:
</p><p>For example,
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/behavior&action=edit&section=2" title="Edit section: Signed overflow">edit</a>]</span> <span class="mw-headline" id="Signed_overflow">Signed overflow</span></h4>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> foo<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">return</span> x<span class="sy2">+</span><span class="nu0">1</span> <span class="sy1">></span> x<span class="sy4">;</span> <span class="co1">// either true or UB due to signed overflow</span>
<span class="br0">}</span></pre></div></div>
<p>may be compiled as (<a rel="nofollow" class="external text" href="https://godbolt.org/g/u1MDjW">demo</a>)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">foo<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">:</span>
movl $<span class="nu19">1</span>, <span class="sy2">%</span>eax
ret</pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/behavior&action=edit&section=3" title="Edit section: Access out of bounds">edit</a>]</span> <span class="mw-headline" id="Access_out_of_bounds">Access out of bounds</span></h4>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> table<span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> exists_in_table<span class="br0">(</span><span class="kw4">int</span> v<span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// return true in one of the first 4 iterations or UB due to out-of-bounds access</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><=</span> <span class="nu0">4</span><span class="sy4">;</span> i<span class="sy2">++</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">if</span> <span class="br0">(</span>table<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1">==</span> v<span class="br0">)</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>May be compiled as (<a rel="nofollow" class="external text" href="https://godbolt.org/g/Mc6Qu1">demo</a>)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">exists_in_table<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">:</span>
movl $<span class="nu19">1</span>, <span class="sy2">%</span>eax
ret</pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/behavior&action=edit&section=4" title="Edit section: Uninitialized scalar">edit</a>]</span> <span class="mw-headline" id="Uninitialized_scalar">Uninitialized scalar</span></h4>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">bool p<span class="sy4">;</span> <span class="co1">// uninitialized local variable</span>
<span class="kw1">if</span><span class="br0">(</span>p<span class="br0">)</span> <span class="co1">// UB access to uninitialized scalar</span>
<a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"p is true"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span><span class="sy3">!</span>p<span class="br0">)</span> <span class="co1">// UB access to uninitialized scalar</span>
<a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"p is false"</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>May produce the following output (observed with an older version of gcc):
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">p is <span class="kw2">true</span>
p is <span class="kw2">false</span></pre></div></div>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><a href="../types/size_t.html"><span class="kw100">size_t</span></a> f<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span>
<span class="br0">{</span>
<a href="../types/size_t.html"><span class="kw100">size_t</span></a> a<span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span>x<span class="br0">)</span> <span class="co1">// either x nonzero or UB</span>
a <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>
<span class="kw1">return</span> a<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<p>May be compiled as (<a rel="nofollow" class="external text" href="https://godbolt.org/g/CZKrdp">demo</a>)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">:</span>
mov eax, <span class="nu0">42</span>
ret</pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/behavior&action=edit&section=5" title="Edit section: Access to pointer passed to realloc">edit</a>]</span> <span class="mw-headline" id="Access_to_pointer_passed_to_realloc">Access to pointer passed to realloc</span></h4>
<div class="t-example"><p>Choose clang to observe the output shown
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include <stdio.h></span>
<span class="co2">#include <stdlib.h></span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</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><a href="../memory/malloc.html"><span class="kw403">malloc</span></a><span class="br0">(</span><span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> <span class="sy2">*</span>q <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><a href="../memory/realloc.html"><span class="kw405">realloc</span></a><span class="br0">(</span>p, <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="sy2">*</span>p <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// UB access to a pointer that was passed to realloc</span>
<span class="sy2">*</span>q <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="kw1">if</span> <span class="br0">(</span>p <span class="sy1">==</span> q<span class="br0">)</span> <span class="co1">// UB access to a pointer that was passed to realloc</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%d%d<span class="es1">\n</span>"</span>, <span class="sy2">*</span>p, <span class="sy2">*</span>q<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">12</pre></div></div>
</div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/behavior&action=edit&section=6" title="Edit section: Infinite loop without side-effects">edit</a>]</span> <span class="mw-headline" id="Infinite_loop_without_side-effects">Infinite loop without side-effects</span></h4>
<div class="t-example"><p>Choose clang to observe the output shown
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include <stdio.h></span>
 
<span class="kw4">int</span> fermat<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw4">const</span> <span class="kw4">int</span> MAX <span class="sy1">=</span> <span class="nu0">1000</span><span class="sy4">;</span>
<span class="kw4">int</span> a<span class="sy1">=</span><span class="nu0">1</span>,b<span class="sy1">=</span><span class="nu0">1</span>,c<span class="sy1">=</span><span class="nu0">1</span><span class="sy4">;</span>
<span class="co1">// Endless loop with no side effects is UB</span>
<span class="kw1">while</span> <span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">if</span> <span class="br0">(</span><span class="br0">(</span><span class="br0">(</span>a<span class="sy2">*</span>a<span class="sy2">*</span>a<span class="br0">)</span> <span class="sy1">==</span> <span class="br0">(</span><span class="br0">(</span>b<span class="sy2">*</span>b<span class="sy2">*</span>b<span class="br0">)</span><span class="sy2">+</span><span class="br0">(</span>c<span class="sy2">*</span>c<span class="sy2">*</span>c<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
a<span class="sy2">++</span><span class="sy4">;</span>
<span class="kw1">if</span> <span class="br0">(</span>a<span class="sy1">></span>MAX<span class="br0">)</span> <span class="br0">{</span> a<span class="sy1">=</span><span class="nu0">1</span><span class="sy4">;</span> b<span class="sy2">++</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">if</span> <span class="br0">(</span>b<span class="sy1">></span>MAX<span class="br0">)</span> <span class="br0">{</span> b<span class="sy1">=</span><span class="nu0">1</span><span class="sy4">;</span> c<span class="sy2">++</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">if</span> <span class="br0">(</span>c<span class="sy1">></span>MAX<span class="br0">)</span> <span class="br0">{</span> c<span class="sy1">=</span><span class="nu0">1</span><span class="sy4">;</span><span class="br0">}</span>
<span class="br0">}</span>
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">if</span> <span class="br0">(</span>fermat<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
<a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"Fermat's Last Theorem has been disproved."</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">else</span>
<a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"Fermat's Last Theorem has not been disproved."</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Fermat's Last Theorem has been disproved.</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/behavior&action=edit&section=7" 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> 3.4 Behavior (p: 3-4)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 4/2 Undefined behavior (p: 8)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 3.4 Behavior (p: 3-4)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 4/2 Undefined behavior (p: 7)
</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> 1.6 DEFINITIONS OF TERMS
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/behavior&action=edit&section=8" title="Edit section: External links">edit</a>]</span> <span class="mw-headline" id="External_links">External links</span></h3>
<ul><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html">What Every C Programmer Should Know About Undefined Behavior #1/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html">What Every C Programmer Should Know About Undefined Behavior #2/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html">What Every C Programmer Should Know About Undefined Behavior #3/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blogs.msdn.com/b/oldnewthing/archive/2014/06/27/10537746.aspx">Undefined behavior can result in time travel (among other things, but time travel is the funkiest)</a>
</li><li> <a rel="nofollow" class="external text" href="http://www.cs.utah.edu/~regehr/papers/overflow12.pdf">Understanding Integer Overflow in C/C++</a>
</li><li> <a rel="nofollow" class="external text" href="http://kukuruku.co/hub/cpp/undefined-behavior-and-fermats-last-theorem">Undefined Behavior and Fermat’s Last Theorem</a>
</li><li> <a rel="nofollow" class="external text" href="http://lwn.net/Articles/342330/">Fun with NULL pointers, part 1</a> (local exploit in Linux 2.6.30 caused by UB due to null pointer dereference)
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/behavior&action=edit&section=9" 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/ub.html" title="cpp/language/ub">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>undefined behavior</span></span></div>
</td></tr>
</table></div>
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