<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><htmlxmlns="http://www.w3.org/1999/xhtml"><head><metahttp-equiv="Content-Type"content="text/html; charset=UTF-8"/><title>Use of errno by the library</title><metaname="generator"content="DocBook XSL-NS Stylesheets V1.78.1"/><metaname="keywords"content="ISO C++, library"/><metaname="keywords"content="ISO C++, runtime, library"/><linkrel="home"href="../index.html"title="The GNU C++ Library"/><linkrel="up"href="diagnostics.html"title="Chapter 5. Diagnostics"/><linkrel="prev"href="diagnostics.html"title="Chapter 5. Diagnostics"/><linkrel="next"href="concept_checking.html"title="Concept Checking"/></head><body><divclass="navheader"><tablewidth="100%"summary="Navigation header"><tr><thcolspan="3"align="center">Use of errno by the library</th></tr><tr><tdwidth="20%"align="left"><aaccesskey="p"href="diagnostics.html">Prev</a></td><thwidth="60%"align="center">Chapter 5.
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The C and POSIX standards guarantee that <codeclass="varname">errno</code>
is never set to zero by any library function.
The C++ standard has less to say about when <codeclass="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 <codeclass="varname">errno</code> on error, beyond what is specified for
functions that come from the C library.
For example, when <codeclass="function">std::stoi</code> throws an exception of
type <codeclass="classname">std::out_of_range</code>, <codeclass="varname">errno</code>
may or may not have been set to <codeclass="constant">ERANGE</code>.
</p><p>
Parts of the C++ library may be implemented in terms of C library
functions, which may result in <codeclass="varname">errno</code> being set
with no explicit call to a C function. For example, on a target where
