fixed SEGFAULTs in InterfaceCurses::vmsg()

 * both vmsg() and stdio_msg() behave like vprintf() are allowed
   to leave their `va_list` in an undefined state.
 * therefore when writing messages to stdio in addition to the
   message line, we have to copy the argument list.
 * fixes SEGFAULTs when trying to log any message
   (but this bug did not manifest on every test system)

2 files changed, 17 insertions, 8 deletions

diff --git a/src/interface-curses.cpp b/src/interface-curses.cpp
index e46af51..c3216ce 100644
--- a/src/interface-curses.cpp
+++ b/src/interface-curses.cpp
@@ -384,20 +384,21 @@ InterfaceCurses::vmsg_impl(MessageType type, const gchar *fmt, va_list ap)
 {
 	attr_t attr;
 
+	if (!msg_window) { /* batch mode */
+		stdio_vmsg(type, fmt, ap);
+		return;
+	}
+
 	/*
 	 * On most platforms we can write to stdout/stderr
 	 * even in interactive mode.
 	 */
 #if defined(XCURSES) || defined(PDCURSES_WIN32A) || \
     defined(NCURSES_UNIX) || defined(NCURSES_WIN32)
-	stdio_vmsg(type, fmt, ap);
-	if (!msg_window) /* batch mode */
-		return;
-#else
-	if (!msg_window) { /* batch mode */
-		stdio_vmsg(type, fmt, ap);
-		return;
-	}
+	va_list aq;
+	va_copy(aq, ap);
+	stdio_vmsg(type, fmt, aq);
+	va_end(aq);
 #endif
 
 	switch (type) {
diff --git a/src/interface.cpp b/src/interface.cpp
index 41cc93d..0bcdf28 100644
--- a/src/interface.cpp
+++ b/src/interface.cpp
@@ -107,6 +107,14 @@ Interface<InterfaceImpl, ViewImpl>::UndoTokenInfoUpdate<Type>::run(void)
 	interface.info_update(obj);
 }
 
+/**
+ * Print a message to the appropriate stdio streams.
+ *
+ * This method has similar semantics to `vprintf`, i.e.
+ * it leaves `ap` undefined. Therefore to pass the format
+ * string and arguments to another `vprintf`-like function,
+ * you have to copy the arguments via `va_copy`.
+ */
 template <class InterfaceImpl, class ViewImpl>
 void
 Interface<InterfaceImpl, ViewImpl>::stdio_vmsg(MessageType type, const gchar *fmt, va_list ap)