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</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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