diff --git a/changes/bug5969_022 b/changes/bug5969_022
new file mode 100644
index 0000000000..57c8744267
--- /dev/null
+++ b/changes/bug5969_022
@@ -0,0 +1,7 @@
+  o Minor bugfixes
+    - Fix a build warning with Clang 3.1 related to our use of vasprint.
+      Fix for bug 5969. Bugfix on 0.2.2.11-alpha.
+
+  o Compilation improvements:
+    - Tell GCC and Clang to check for any errors in format strings passed
+      to the tor_v*(print|scan)f functions.
diff --git a/src/common/compat.h b/src/common/compat.h
index d2f1fd1295..fc70caf50c 100644
--- a/src/common/compat.h
+++ b/src/common/compat.h
@@ -261,11 +261,12 @@ void tor_munmap_file(tor_mmap_t *handle) ATTR_NONNULL((1));
 int tor_snprintf(char *str, size_t size, const char *format, ...)
   CHECK_PRINTF(3,4) ATTR_NONNULL((1,3));
 int tor_vsnprintf(char *str, size_t size, const char *format, va_list args)
-  ATTR_NONNULL((1,3));
+  CHECK_PRINTF(3,0) ATTR_NONNULL((1,3));
 
 int tor_asprintf(char **strp, const char *fmt, ...)
   CHECK_PRINTF(2,3);
-int tor_vasprintf(char **strp, const char *fmt, va_list args);
+int tor_vasprintf(char **strp, const char *fmt, va_list args)
+  CHECK_PRINTF(2,0);
 
 const void *tor_memmem(const void *haystack, size_t hlen, const void *needle,
                        size_t nlen)  ATTR_PURE ATTR_NONNULL((1,3));
diff --git a/src/common/util.h b/src/common/util.h
index b9db25ca73..d4771562ee 100644
--- a/src/common/util.h
+++ b/src/common/util.h
@@ -211,7 +211,11 @@ const char *escaped(const char *string);
 struct smartlist_t;
 void wrap_string(struct smartlist_t *out, const char *string, size_t width,
                  const char *prefix0, const char *prefixRest);
-int tor_vsscanf(const char *buf, const char *pattern, va_list ap);
+int tor_vsscanf(const char *buf, const char *pattern, va_list ap)
+#ifdef __GNUC__
+  __attribute__((format(scanf, 2, 0)))
+#endif
+  ;
 int tor_sscanf(const char *buf, const char *pattern, ...)
 #ifdef __GNUC__
   __attribute__((format(scanf, 2, 3)))