diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 97daa79d06b81d967fe3b9fbe40ffcc683039a25..378ffa484e17ed4c69d29ed421978f4afb4fc81a 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,11 @@
+2016-11-30  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c/78498
+	* selftest.c (selftest::assert_strndup_eq): New function.
+	(selftest::test_strndup): New function.
+	(selftest::test_libiberty): New function.
+	(selftest::selftest_c_tests): Call test_libiberty.
+
 2016-11-30  Segher Boessenkool  <segher@kernel.crashing.org>
 
 	PR rtl-optimization/78610
diff --git a/gcc/selftest.c b/gcc/selftest.c
index 2a729be9b5e59637d1ffd2520584d3c842b1aa33..6df73c24084ab1ee11bdc4888d6c75a539add4a6 100644
--- a/gcc/selftest.c
+++ b/gcc/selftest.c
@@ -198,6 +198,53 @@ read_file (const location &loc, const char *path)
   return result;
 }
 
+/* Selftests for libiberty.  */
+
+/* Verify that both strndup and xstrndup generate EXPECTED
+   when called on SRC and N.  */
+
+static void
+assert_strndup_eq (const char *expected, const char *src, size_t n)
+{
+  char *buf = strndup (src, n);
+  if (buf)
+    ASSERT_STREQ (expected, buf);
+  free (buf);
+
+  buf = xstrndup (src, n);
+  ASSERT_STREQ (expected, buf);
+  free (buf);
+}
+
+/* Verify that strndup and xstrndup work as expected.  */
+
+static void
+test_strndup ()
+{
+  assert_strndup_eq ("", "test", 0);
+  assert_strndup_eq ("t", "test", 1);
+  assert_strndup_eq ("te", "test", 2);
+  assert_strndup_eq ("tes", "test", 3);
+  assert_strndup_eq ("test", "test", 4);
+  assert_strndup_eq ("test", "test", 5);
+
+  /* Test on an string without zero termination.  */
+  const char src[4] = {'t', 'e', 's', 't'};
+  assert_strndup_eq ("", src, 0);
+  assert_strndup_eq ("t", src, 1);
+  assert_strndup_eq ("te", src, 2);
+  assert_strndup_eq ("tes", src, 3);
+  assert_strndup_eq ("test", src, 4);
+}
+
+/* Run selftests for libiberty.  */
+
+static void
+test_libiberty ()
+{
+  test_strndup ();
+}
+
 /* Selftests for the selftest system itself.  */
 
 /* Sanity-check the ASSERT_ macros with various passing cases.  */
@@ -245,6 +292,7 @@ test_read_file ()
 void
 selftest_c_tests ()
 {
+  test_libiberty ();
   test_assertions ();
   test_named_temp_file ();
   test_read_file ();
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index d1b5c90a7cac8d77996b2135a4d44b50960dbfb6..d6166b9edde498a0298a852b7c9351bb2ee18fb2 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2016-11-30  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c/78498
+	* gcc.dg/format/pr78494.c: New test case.
+
 2016-11-30  David Edelsohn  <dje.gcc@gmail.com>
 
 	* g++.dg/debug/dwarf2/ref-1.C: Don't XFAIL scan-assembler-not on AIX.
diff --git a/gcc/testsuite/gcc.dg/format/pr78494.c b/gcc/testsuite/gcc.dg/format/pr78494.c
new file mode 100644
index 0000000000000000000000000000000000000000..4b53a68c2c04e53031ee146c7e66da3da17a0a98
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/format/pr78494.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wall -Wextra -fdiagnostics-show-caret" } */
+
+void f (void)
+{
+  __builtin_printf ("%i", ""); /* { dg-warning "expects argument of type" } */
+/* { dg-begin-multiline-output "" }
+   __builtin_printf ("%i", "");
+                      ~^   ~~
+                      %s
+   { dg-end-multiline-output "" } */
+}
diff --git a/libiberty/ChangeLog b/libiberty/ChangeLog
index 42164c406af991c6d5eb75e075b67fba35e7286f..f0959c99f5cdc3afab526b6c7d12567f7c741328 100644
--- a/libiberty/ChangeLog
+++ b/libiberty/ChangeLog
@@ -1,3 +1,11 @@
+2016-11-30  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c/78498
+	* strndup.c (strlen): Delete decl.
+	(strnlen): Add decl.
+	(strndup): Call strnlen rather than strlen.
+	* xstrndup.c (xstrndup): Likewise.
+
 2016-11-29  Nathan Sidwell  <nathan@acm.org>
 
 	* cp-demangle.c (d_print_comp_inner): Fix parameter indentation.
diff --git a/libiberty/strndup.c b/libiberty/strndup.c
index 9e9b4e2991f833082acd176f48825f76d5a888b9..4556b9627467f584670272bb9ebd073cd943afb0 100644
--- a/libiberty/strndup.c
+++ b/libiberty/strndup.c
@@ -33,7 +33,7 @@ memory was available.  The result is always NUL terminated.
 #include "ansidecl.h"
 #include <stddef.h>
 
-extern size_t	strlen (const char*);
+extern size_t	strnlen (const char *s, size_t maxlen);
 extern PTR	malloc (size_t);
 extern PTR	memcpy (PTR, const PTR, size_t);
 
@@ -41,10 +41,7 @@ char *
 strndup (const char *s, size_t n)
 {
   char *result;
-  size_t len = strlen (s);
-
-  if (n < len)
-    len = n;
+  size_t len = strnlen (s, n);
 
   result = (char *) malloc (len + 1);
   if (!result)
diff --git a/libiberty/xstrndup.c b/libiberty/xstrndup.c
index 0a41f608ec0b9c71ddebc4975c688c88bdde30a3..c3d2d833902eac606548ac6e64821da74411841e 100644
--- a/libiberty/xstrndup.c
+++ b/libiberty/xstrndup.c
@@ -48,10 +48,7 @@ char *
 xstrndup (const char *s, size_t n)
 {
   char *result;
-  size_t len = strlen (s);
-
-  if (n < len)
-    len = n;
+  size_t len = strnlen (s, n);
 
   result = XNEWVEC (char, len + 1);