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+    <h1 id="firstHeading" class="firstHeading">errno</h1>            <table class="t-dcl-begin"> <tr class="t-dsc-header"> <th> Defined in header <code>&lt;errno.h&gt;</code> </th> <th> </th> <th> </th> </tr> <tr class="t-dcl"> <td class="t-dcl-nopad"> <pre data-language="c">#define errno /*implementation-defined*/</pre>
+</td> <td class="t-dcl-nopad"> </td> <td class="t-dcl-nopad"> </td> </tr>  </table> <p><code>errno</code> is a preprocessor macro (but see note below) that expands to a <span class="t-rev-inl t-since-c11"><span>thread-local</span><span><span class="t-mark-rev t-since-c11">(since C11)</span></span></span> modifiable lvalue of type <code>int</code>. Several standard library functions indicate errors by writing positive integers to <code>errno</code>. Typically, the value of <code>errno</code> is set to one of the error codes listed in <code>&lt;errno.h&gt;</code> as macro constants beginning with the letter <code>E</code> followed by uppercase letters or digits.</p>
+<p>The value of <code>errno</code> is <code>​0​</code> at program startup, and although library functions are allowed to write positive integers to <code>errno</code> whether or not an error occurred, library functions never store <code>​0​</code> in <code>errno</code>.</p>
+<p>Library functions <code><a href="../io/perror" title="c/io/perror">perror</a></code> and <code><a href="../string/byte/strerror" title="c/string/byte/strerror">strerror</a></code> can be used to obtain textual descriptions of the error conditions that correspond to the current <code>errno</code> value.</p>
+<p>Note: Until C11, the C standards had contradictory requirements, in that they said that <code>errno</code> is a macro but <i>also</i> that "it is unspecified whether <code>errno</code> is a macro or an identifier declared with external linkage". C11 fixes this, requiring that it be defined as a macro (see also WG14 <a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1338.htm">N1338</a>).</p>
+<h3 id="Example"> Example</h3> <div class="t-example"> <div class="c source-c"><pre data-language="c">#include &lt;stdio.h&gt;
+#include &lt;math.h&gt;
+#include &lt;errno.h&gt;
+ 
+void show_errno(void)
+{
+    const char *err_info = "unknown error";
+    switch (errno) {
+    case EDOM:
+        err_info = "domain error";
+        break;
+    case EILSEQ:
+        err_info = "illegal sequence";
+        break;
+    case ERANGE:
+        err_info = "pole or range error";
+        break;
+    case 0:
+        err_info = "no error";
+    }
+    fputs(err_info, stdout);
+    puts(" occurred");
+}
+ 
+int main(void)
+{
+    fputs("MATH_ERRNO is ", stdout);
+    puts(math_errhandling &amp; MATH_ERRNO ? "set" : "not set");
+ 
+    errno = 0;
+    1.0/0.0;
+    show_errno();
+ 
+    errno = 0;
+    acos(+1.1);
+    show_errno();
+ 
+    errno = 0;
+    log(0.0);
+    show_errno();
+ 
+    errno = 0;
+    sin(0.0);
+    show_errno();
+}</pre></div> <p>Possible output:</p>
+<div class="text source-text"><pre data-language="c">MATH_ERRNO is set
+no error occurred
+domain error occurred
+pole or range error occurred
+no error occurred</pre></div> </div> <h3 id="References"> References</h3>  <ul>
+<li> C11 standard (ISO/IEC 9899:2011): </li>
+<ul>
+<li> 7.5 Errors &lt;errno.h&gt; (p: 205) </li>
+<li> K.3.1.3 Use of errno (p: 584) </li>
+<li> K.3.2 Errors &lt;errno.h&gt; (p: 585) </li>
+</ul>
+<li> C99 standard (ISO/IEC 9899:1999): </li>
+<ul><li> 7.5 Errors &lt;errno.h&gt; (p: 186) </li></ul>
+<li> C89/C90 standard (ISO/IEC 9899:1990): </li>
+<ul><li> 4.1.3 Errors &lt;errno.h&gt; </li></ul>
+</ul>           <h3 id="See_also"> See also</h3> <table class="t-dsc-begin"> <tr class="t-dsc"> <td> <div><a href="errno_macros" title="c/error/errno macros"> <span class="t-lines"><span>E2BIG, EACCES, ..., EXDEV</span></span></a></div> </td> <td> macros for standard POSIX-compatible error conditions <br> <span class="t-mark">(macro constant)</span>  </td>
+</tr> <tr class="t-dsc"> <td> <div><a href="../io/perror" title="c/io/perror"> <span class="t-lines"><span>perror</span></span></a></div> </td> <td> displays a character string corresponding of the current error to <code><a href="../io/std_streams" title="c/io/std streams">stderr</a></code> <br> <span class="t-mark">(function)</span>  </td>
+</tr> <tr class="t-dsc"> <td> <div><a href="../string/byte/strerror" title="c/string/byte/strerror"> <span class="t-lines"><span>strerror</span><span>strerror_s</span><span>strerrorlen_s</span></span></a></div>
+<div><span class="t-lines"><span><span class="t-mark-rev t-since-c11">(C11)</span></span><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div> </td> <td> returns a text version of a given error code <br> <span class="t-mark">(function)</span>  </td>
+</tr> <tr class="t-dsc"> <td> <div><a href="../numeric/math/math_errhandling" title="c/numeric/math/math errhandling"> <span class="t-lines"><span>math_errhandling</span><span>MATH_ERRNO</span><span>MATH_ERREXCEPT</span></span></a></div>
+<div><span class="t-lines"><span><span class="t-mark-rev t-since-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div> </td> <td> defines the error handling mechanism used by the common mathematical functions <br> <span class="t-mark">(macro constant)</span>  </td>
+</tr> <tr class="t-dsc"> <td colspan="2"> <span><a href="https://en.cppreference.com/w/cpp/error/errno" title="cpp/error/errno">C++ documentation</a></span> for <code>errno</code> </td>
+</tr> </table>            <div class="_attribution">
+  <p class="_attribution-p">
+    &copy; cppreference.com<br>Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.<br>
+    <a href="https://en.cppreference.com/w/c/error/errno" class="_attribution-link">https://en.cppreference.com/w/c/error/errno</a>
+  </p>
+</div>