diff --git a/xen/lib/strcmp.c b/xen/lib/strcmp.c
index 465f1c4191..f85c1e8741 100644
--- a/xen/lib/strcmp.c
+++ b/xen/lib/strcmp.c
@@ -11,14 +11,16 @@
  */
 int (strcmp)(const char *cs, const char *ct)
 {
-	register signed char __res;
+	unsigned char *csu = (unsigned char *)cs;
+	unsigned char *ctu = (unsigned char *)ct;
+	int res;
 
 	while (1) {
-		if ((__res = *cs - *ct++) != 0 || !*cs++)
+		if ((res = *csu - *ctu++) != 0 || !*csu++)
 			break;
 	}
 
-	return __res;
+	return res;
 }
 
 /*
diff --git a/xen/lib/strncmp.c b/xen/lib/strncmp.c
index 9af7fa1c99..1480f58c2e 100644
--- a/xen/lib/strncmp.c
+++ b/xen/lib/strncmp.c
@@ -12,15 +12,17 @@
  */
 int (strncmp)(const char *cs, const char *ct, size_t count)
 {
-	register signed char __res = 0;
+	unsigned char *csu = (unsigned char *)cs;
+	unsigned char *ctu = (unsigned char *)ct;
+	int res = 0;
 
 	while (count) {
-		if ((__res = *cs - *ct++) != 0 || !*cs++)
+		if ((res = *csu - *ctu++) != 0 || !*csu++)
 			break;
 		count--;
 	}
 
-	return __res;
+	return res;
 }
 
 /*