diff --git a/ChangeLog b/ChangeLog
index a5b1620d7b..4f9b309137 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Tue Nov  4 12:51:31 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* include/ruby/ruby.h (PRIsVALUE), vsnprintf.c (BSD_vfprintf): add
+	  RUBY_PRI_VALUE_MARK to reduce danger of accidental conflict with
+	  plain "%i".  binary incompatible with extension libraries using
+	  PRIsVALUE and built for 2.1 and earlier.  [EXPERIMENTAL]
+
 Tue Nov  4 12:33:44 2014  SHIBATA Hiroshi  <shibata.hiroshi@gmail.com>
 
 	* object.c: fix document of Kernel#String by @suzukaze
diff --git a/include/ruby/ruby.h b/include/ruby/ruby.h
index 2b175fd89e..18183f787c 100644
--- a/include/ruby/ruby.h
+++ b/include/ruby/ruby.h
@@ -123,20 +123,21 @@ typedef char ruby_check_sizeof_voidp[SIZEOF_VOIDP == sizeof(void*) ? 1 : -1];
 #define PRI_64_PREFIX PRI_LL_PREFIX
 #endif
 
+#define RUBY_PRI_VALUE_MARK "\v"
 #if defined PRIdPTR && !defined PRI_VALUE_PREFIX
 #define PRIdVALUE PRIdPTR
 #define PRIoVALUE PRIoPTR
 #define PRIuVALUE PRIuPTR
 #define PRIxVALUE PRIxPTR
 #define PRIXVALUE PRIXPTR
-#define PRIsVALUE PRIiPTR
+#define PRIsVALUE PRIiPTR""RUBY_PRI_VALUE_MARK
 #else
 #define PRIdVALUE PRI_VALUE_PREFIX"d"
 #define PRIoVALUE PRI_VALUE_PREFIX"o"
 #define PRIuVALUE PRI_VALUE_PREFIX"u"
 #define PRIxVALUE PRI_VALUE_PREFIX"x"
 #define PRIXVALUE PRI_VALUE_PREFIX"X"
-#define PRIsVALUE PRI_VALUE_PREFIX"i"
+#define PRIsVALUE PRI_VALUE_PREFIX"i"RUBY_PRI_VALUE_MARK
 #endif
 #ifndef PRI_VALUE_PREFIX
 # define PRI_VALUE_PREFIX ""
diff --git a/sprintf.c b/sprintf.c
index 85adc216f3..d370b8d8de 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -1253,6 +1253,9 @@ fmt_setup(char *buf, size_t size, int c, int flags, int width, int prec)
 #define FLOATING_POINT 1
 #define BSD__dtoa ruby_dtoa
 #define BSD__hdtoa ruby_hdtoa
+#ifdef RUBY_PRI_VALUE_MARK
+# define PRI_EXTRA_MARK RUBY_PRI_VALUE_MARK
+#endif
 #include "vsnprintf.c"
 
 typedef struct {
diff --git a/vsnprintf.c b/vsnprintf.c
index f8b5a8970b..36f6f25465 100644
--- a/vsnprintf.c
+++ b/vsnprintf.c
@@ -806,7 +806,21 @@ reswitch:	switch (ch) {
 #else
 # define INTPTR_FLAG 0
 #endif
-			if (fp->vextra && (flags & INTPTR_MASK) == INTPTR_FLAG) {
+#ifdef PRI_EXTRA_MARK
+# define PRI_EXTRA_MARK_LEN (sizeof(PRI_EXTRA_MARK)-1)
+# define IS_PRI_EXTRA_MARK(s) \
+	(PRI_EXTRA_MARK_LEN < 1 || \
+	 (*(s) == PRI_EXTRA_MARK[0] && \
+	  (PRI_EXTRA_MARK_LEN == 1 || \
+	   strncmp((s)+1, PRI_EXTRA_MARK+1, \
+		   PRI_EXTRA_MARK_LEN-1) == 0)))
+#else
+# define PRI_EXTRA_MARK_LEN 0
+# define IS_PRI_EXTRA_MARK(s) 1
+#endif
+			if (fp->vextra && (flags & INTPTR_MASK) == INTPTR_FLAG &&
+			    IS_PRI_EXTRA_MARK(fmt)) {
+				fmt += PRI_EXTRA_MARK_LEN;
 				FLUSH();
 #if defined _HAVE_SANE_QUAD_ && SIZEOF_VOIDP == SIZEOF_LONG_LONG
 				uqval = va_arg(ap, u_quad_t);