diff --git a/json_util.c b/json_util.c
index af6f3d6..06e4a71 100644
--- a/json_util.c
+++ b/json_util.c
@@ -52,7 +52,7 @@
 #include "json_tokener.h"
 #include "json_util.h"
 
-struct json_object* json_object_from_file(char *filename)
+struct json_object* json_object_from_file(const char *filename)
 {
   struct printbuf *pb;
   struct json_object *obj;
diff --git a/json_util.h b/json_util.h
index 6ab0287..134390f 100644
--- a/json_util.h
+++ b/json_util.h
@@ -21,7 +21,7 @@ extern "C" {
 #define JSON_FILE_BUF_SIZE 4096
 
 /* utility functions */
-extern struct json_object* json_object_from_file(char *filename);
+extern struct json_object* json_object_from_file(const char *filename);
 extern int json_object_to_file(char *filename, struct json_object *obj);
 
 #ifdef __cplusplus