diff --git a/doc/developer.texi b/doc/developer.texi
index 1c29ae0e14..c11a5ef7d0 100644
--- a/doc/developer.texi
+++ b/doc/developer.texi
@@ -169,6 +169,14 @@ For externally visible symbols, each library has its own prefix. Check
 the existing code and choose names accordingly.
 @end itemize
 
+Furthermore, name space reserved for the system should not be invaded.
+Identifiers ending in @code{_t} are reserved by
+@url{http://pubs.opengroup.org/onlinepubs/007904975/functions/xsh_chap02_02.html#tag_02_02_02, POSIX}.
+Also avoid names starting with @code{__} or @code{_} followed by an uppercase
+letter as they are reserved by the C standard. Names starting with @code{_}
+are reserved at the file level and may not be used for externally visible
+symbols. If in doubt, just avoid names starting with @code{_} altogether.
+
 @subsection Miscellaneous conventions
 @itemize @bullet
 @item
diff --git a/libavutil/internal.h b/libavutil/internal.h
index 462f08c44c..2dacb2c5af 100644
--- a/libavutil/internal.h
+++ b/libavutil/internal.h
@@ -60,7 +60,7 @@
 
 // Some broken preprocessors need a second expansion
 // to be forced to tokenize __VA_ARGS__
-#define EXPAND(x) x
+#define E1(x) x
 
 #define LOCAL_ALIGNED_A(a, t, v, s, o, ...)             \
     uint8_t la_##v[sizeof(t s o) + (a)];                \
@@ -70,16 +70,16 @@
     DECLARE_ALIGNED(a, t, la_##v) s o;                  \
     t (*v) o = la_##v
 
-#define LOCAL_ALIGNED(a, t, v, ...) EXPAND(LOCAL_ALIGNED_A(a, t, v, __VA_ARGS__,,))
+#define LOCAL_ALIGNED(a, t, v, ...) E1(LOCAL_ALIGNED_A(a, t, v, __VA_ARGS__,,))
 
 #if HAVE_LOCAL_ALIGNED_8
-#   define LOCAL_ALIGNED_8(t, v, ...) EXPAND(LOCAL_ALIGNED_D(8, t, v, __VA_ARGS__,,))
+#   define LOCAL_ALIGNED_8(t, v, ...) E1(LOCAL_ALIGNED_D(8, t, v, __VA_ARGS__,,))
 #else
 #   define LOCAL_ALIGNED_8(t, v, ...) LOCAL_ALIGNED(8, t, v, __VA_ARGS__)
 #endif
 
 #if HAVE_LOCAL_ALIGNED_16
-#   define LOCAL_ALIGNED_16(t, v, ...) EXPAND(LOCAL_ALIGNED_D(16, t, v, __VA_ARGS__,,))
+#   define LOCAL_ALIGNED_16(t, v, ...) E1(LOCAL_ALIGNED_D(16, t, v, __VA_ARGS__,,))
 #else
 #   define LOCAL_ALIGNED_16(t, v, ...) LOCAL_ALIGNED(16, t, v, __VA_ARGS__)
 #endif