commit 502928c8061343e82e982e06299c11d465f64b6c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 30 14:10:58 2015 +0100

    Save-and-restore errno more carefully in libstdc++
    
    	* doc/xml/manual/diagnostics.xml: Document use of errno.
    	* config/locale/generic/c_locale.cc (_Save_errno): New helper.
    	(__convert_to_v): Use _Save_errno.
    	* include/ext/string_conversions.h (__stoa): Only restore errno when
    	it isn't set to non-zero.

diff --git a/libstdc++-v3/config/locale/generic/c_locale.cc b/libstdc++-v3/config/locale/generic/c_locale.cc
index 6da5f22..8dfea6b 100644
--- a/libstdc++-v3/config/locale/generic/c_locale.cc
+++ b/libstdc++-v3/config/locale/generic/c_locale.cc
@@ -44,6 +44,16 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+  namespace
+  {
+    struct _Save_errno
+    {
+      _Save_errno() : _M_errno(errno) { errno = 0; }
+      ~_Save_errno() { if (errno == 0) errno = _M_errno; }
+      int _M_errno;
+    };
+  }
+
   template<>
     void
     __convert_to_v(const char* __s, float& __v, ios_base::iostate& __err,
@@ -59,7 +69,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       bool __overflow = false;
 
 #if !__FLT_HAS_INFINITY__
-      errno = 0;
+      const _Save_errno __save_errno;
 #endif
 
 #ifdef _GLIBCXX_HAVE_STRTOF
@@ -123,7 +133,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       char* __sanity;
 
 #if !__DBL_HAS_INFINITY__
-      errno = 0;
+      const _Save_errno __save_errno;
 #endif
 
       __v = strtod(__s, &__sanity);
@@ -167,7 +177,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       setlocale(LC_ALL, "C");
 
 #if !__LDBL_HAS_INFINITY__
-      errno = 0;
+      const _Save_errno __save_errno;
 #endif
 
 #if defined(_GLIBCXX_HAVE_STRTOLD) && !defined(_GLIBCXX_HAVE_BROKEN_STRTOLD)
diff --git a/libstdc++-v3/doc/xml/manual/diagnostics.xml b/libstdc++-v3/doc/xml/manual/diagnostics.xml
index 88ed2e2..3ceb5b3 100644
--- a/libstdc++-v3/doc/xml/manual/diagnostics.xml
+++ b/libstdc++-v3/doc/xml/manual/diagnostics.xml
@@ -71,6 +71,38 @@
   </section>
 </section>
 
+<section xml:id="std.diagnostics.errno" xreflabel="errno"><info><title>Use of errno by the library</title></info>
+
+  <para>
+    The C and POSIX standards guarantee that <varname>errno</varname>
+    is never set to zero by any library function.
+    The C++ standard has less to say about when <varname>errno</varname>
+    is or isn't set, but libstdc++ follows the same rule and never sets
+    it to zero.
+  </para>
+
+  <para>
+    On the other hand, there are few guarantees about when the C++ library
+    sets <varname>errno</varname> on error, beyond what is specified for
+    functions that come from the C library.
+    For example, when <function>std::stoi</function> throws an exception of
+    type <classname>std::out_of_range</classname>, <varname>errno</varname>
+    may or may not have been set to <constant>ERANGE</constant>.
+  </para>
+
+  <para>
+    Parts of the C++ library may be implemented in terms of C library
+    functions, which may result in <varname>errno</varname> being set
+    with no explicit call to a C function. For example, on a target where
+    <function>operator new</function> uses <function>malloc</function>
+    a failed memory allocation with <function>operator new</function> might
+    set <varname>errno</varname> to <constant>ENOMEM</constant>.
+    Which C++ library functions can set <varname>errno</varname> in this way
+    is unspecified because it may vary between platforms and between releases.
+  </para>
+
+</section>
+
 <section xml:id="std.diagnostics.concept_checking" xreflabel="Concept Checking"><info><title>Concept Checking</title></info>
   <?dbhtml filename="concept_checking.html"?>
   
diff --git a/libstdc++-v3/include/ext/string_conversions.h b/libstdc++-v3/include/ext/string_conversions.h
index 58387a2..7f37e69 100644
--- a/libstdc++-v3/include/ext/string_conversions.h
+++ b/libstdc++-v3/include/ext/string_conversions.h
@@ -58,8 +58,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _Ret __ret;
 
       _CharT* __endptr;
-      const int __saved_errno = errno;
-      errno = 0;
+
+      struct _Save_errno {
+	_Save_errno() : _M_errno(errno) { errno = 0; }
+	~_Save_errno() { if (errno == 0) errno = _M_errno; }
+	int _M_errno;
+      } const __save_errno;
+
       const _TRet __tmp = __convf(__str, &__endptr, __base...);
 
       if (__endptr == __str)
@@ -71,7 +76,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	std::__throw_out_of_range(__name);
       else
 	__ret = __tmp;
-      errno = __saved_errno;
 
       if (__idx)
 	*__idx = __endptr - __str;