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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Use of errno by the library</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="diagnostics.html" title="Chapter 5.  Diagnostics" /><link rel="prev" href="diagnostics.html" title="Chapter 5.  Diagnostics" /><link rel="next" href="concept_checking.html" title="Concept Checking" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Use of errno by the library</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="diagnostics.html">Prev</a> </td><th width="60%" align="center">Chapter 5. 
  Diagnostics
  
</th><td width="20%" align="right"> <a accesskey="n" href="concept_checking.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.diagnostics.errno"></a>Use of errno by the library</h2></div></div></div><p>
    The C and POSIX standards guarantee that <code class="varname">errno</code>
    is never set to zero by any library function.
    The C++ standard has less to say about when <code class="varname">errno</code>
    is or isn't set, but libstdc++ follows the same rule and never sets
    it to zero.
  </p><p>
    On the other hand, there are few guarantees about when the C++ library
    sets <code class="varname">errno</code> on error, beyond what is specified for
    functions that come from the C library.
    For example, when <code class="function">std::stoi</code> throws an exception of
    type <code class="classname">std::out_of_range</code>, <code class="varname">errno</code>
    may or may not have been set to <code class="constant">ERANGE</code>.
  </p><p>
    Parts of the C++ library may be implemented in terms of C library
    functions, which may result in <code class="varname">errno</code> being set
    with no explicit call to a C function. For example, on a target where
    <code class="function">operator new</code> uses <code class="function">malloc</code>
    a failed memory allocation with <code class="function">operator new</code> might
    set <code class="varname">errno</code> to <code class="constant">ENOMEM</code>.
    Which C++ library functions can set <code class="varname">errno</code> in this way
    is unspecified because it may vary between platforms and between releases.
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  Diagnostics
  
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