doc: Document differences in bitcoind and bitcoin-qt locale handling

src/qt/bitcoin.cpp

@@ -201,6 +201,7 @@ BitcoinApplication::BitcoinApplication(interfaces::Node& node):
  returnValue(0),
  platformStyle(nullptr)
 {
+ // Qt runs setlocale(LC_ALL, "") on initialization.
  RegisterMetaTypes();
  setQuitOnLastWindowClosed(false);
 }

test/lint/lint-locale-dependence.sh

@@ -4,6 +4,39 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 export LC_ALL=C
+
+# Be aware that bitcoind and bitcoin-qt differ in terms of localization: Qt
+# opts in to POSIX localization by running setlocale(LC_ALL, "") on startup,
+# whereas no such call is made in bitcoind.
+#
+# Qt runs setlocale(LC_ALL, "") on initialization. This installs the locale
+# specified by the user's LC_ALL (or LC_*) environment variable as the new
+# C locale.
+#
+# In contrast, bitcoind does not opt in to localization -- no call to
+# setlocale(LC_ALL, "") is made and the environment variables LC_* are
+# thus ignored.
+#
+# This results in situations where bitcoind is guaranteed to be running
+# with the classic locale ("C") whereas the locale of bitcoin-qt will vary
+# depending on the user's environment variables.
+#
+# An example: Assuming the environment variable LC_ALL=de_DE then the
+# call std::to_string(1.23) will return "1.230000" in bitcoind but
+# "1,230000" in bitcoin-qt.
+#
+# From the Qt documentation:
+# "On Unix/Linux Qt is configured to use the system locale settings by default.
+# This can cause a conflict when using POSIX functions, for instance, when
+# converting between data types such as floats and strings, since the notation
+# may differ between locales. To get around this problem, call the POSIX function
+# setlocale(LC_NUMERIC,"C") right after initializing QApplication, QGuiApplication
+# or QCoreApplication to reset the locale that is used for number formatting to
+# "C"-locale."
+#
+# See https://doc.qt.io/qt-5/qcoreapplication.html#locale-settings and
+# https://stackoverflow.com/a/34878283 for more details.
+
 KNOWN_VIOLATIONS=(
  "src/bitcoin-tx.cpp.*stoul"
  "src/bitcoin-tx.cpp.*trim_right"