diff --git a/util.c b/util.c
index e64a93d..43d6224 100644
--- a/util.c
+++ b/util.c
@@ -190,7 +190,7 @@ char *portaddr2str(struct PortAddress *addr)
 	return buf;
 }
 
-clockid_t posix_clock_open(char *device, int *phc_index)
+clockid_t posix_clock_open(const char *device, int *phc_index)
 {
 	struct sk_ts_info ts_info;
 	char phc_device[19];
diff --git a/util.h b/util.h
index 60d28ac..11e0935 100644
--- a/util.h
+++ b/util.h
@@ -116,7 +116,7 @@ char *portaddr2str(struct PortAddress *addr);
  * @param phc_index  Returns the PHC index, if any.
  * @return           A valid clock ID on success or CLOCK_INVALID otherwise.
  */
-clockid_t posix_clock_open(char *device, int *phc_index);
+clockid_t posix_clock_open(const char *device, int *phc_index);
 
 /**
  * Compare two port identities for equality.