diff --git a/lib/util.c b/lib/util.c
index 6a569b6..0397f1e 100644
--- a/lib/util.c
+++ b/lib/util.c
@@ -23,7 +23,7 @@ void *xmalloc(size_t sz)
 }
 
 /* Like strdup, but oom() instead of NULL */
-char *xstrdup(char *s)
+char *xstrdup(const char *s)
 {
     char *d = strdup(s);
     if (!d)
diff --git a/lib/util.h b/lib/util.h
index fdf1f13..b7598e8 100644
--- a/lib/util.h
+++ b/lib/util.h
@@ -2,7 +2,7 @@
 
 void *xmalloc(size_t sz);
 void *xrealloc(void *p, size_t sz);
-char *xstrdup(char *src);
+char *xstrdup(const char *src);
 
 #define new(p) ((p) = xmalloc(sizeof(*(p))))