From e802cf14e30a4f8ca86d8c7e5cb6f3196684099e Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Sat, 10 Feb 2018 07:01:59 -0800
Subject: [PATCH] Add note about errno to the documentation

---
 doc/index.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/doc/index.rst b/doc/index.rst
index ce9b7bf9..a00828b0 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -143,6 +143,12 @@ its numeric value being written to the stream (i.e. 1070 instead of letter 'ю'
 which is represented by ``L'\x42e'`` if we use Unicode) which is rarely what is
 needed.
 
+Note that fmt does not use the value of the ``errno`` global to communicate
+errors to the user, but it may call system functions which set ``errno``. Since
+fmt does not attempt to preserve the value of ``errno``, users should not make
+any assumptions about it and always set it to ``0`` before making any system
+calls that convey error information via ``errno``.
+
 .. _portability:
 
 Portability