the last parameter of XChangeProperty is the number of elements.. and when the element format is 32.. the element is "long" so we have 5 long elements here.

Yes this seems confusing as on mac+linux Long is either 32 or 64bits depending on the architecture, but this is how the X11 protocol is defined. Thus 5 is the correct value for the nelts here.  Not 5 or 10 depending on the architecture.

More info on the confusion https://bugs.freedesktop.org/show_bug.cgi?id=16802

visualtest/include/SDL_visualtest_mischelper.h

@@ -0,0 +1,26 @@
+/**
+ * \file mischelper.c 
+ *
+ * Header with miscellaneous helper functions.
+ */
+
+#ifndef _SDL_visualtest_mischelper_h
+#define _SDL_visualtest_mischelper_h
+
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Stores a 32 digit hexadecimal string representing the MD5 hash of the
+ * string \c str in \c hash.
+ */
+void SDLVisualTest_HashString(char* str, char hash[33]);
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _SDL_visualtest_mischelper_h */