diff --git a/smallapp/unbound-checkconf.c b/smallapp/unbound-checkconf.c
index 7acc0865b..83f84aafe 100644
--- a/smallapp/unbound-checkconf.c
+++ b/smallapp/unbound-checkconf.c
@@ -248,18 +248,18 @@ fname_after_chroot(const char* fname, struct config_file* cfg, int use_chdir)
 	} else if(cfg->directory && cfg->directory[0]) {
 		/* prepend chdir */
 		if(slashit && cfg->directory[0] != '/')
-			strncat(buf, "/", sizeof(buf)-1);
+			strncat(buf, "/", sizeof(buf)-strlen(buf)-1);
 		if(strncmp(cfg->chrootdir, cfg->directory, 
 			strlen(cfg->chrootdir)) == 0)
 			strncat(buf, cfg->directory+strlen(cfg->chrootdir), 
-				sizeof(buf)-1);
-		else strncat(buf, cfg->directory, sizeof(buf)-1);
+				   sizeof(buf)-strlen(buf)-1);
+		else strncat(buf, cfg->directory, sizeof(buf)-strlen(buf)-1);
 		slashit = 1;
 	}
 	/* fname */
 	if(slashit && fname[0] != '/')
-		strncat(buf, "/", sizeof(buf)-1);
-	strncat(buf, fname, sizeof(buf)-1);
+		strncat(buf, "/", sizeof(buf)-strlen(buf)-1);
+	strncat(buf, fname, sizeof(buf)-strlen(buf)-1);
 	buf[sizeof(buf)-1] = 0;
 	return buf;
 }
@@ -329,9 +329,9 @@ morechecks(struct config_file* cfg, char* fname)
 		if(fname[0] != '/') {
 			if(getcwd(buf, sizeof(buf)) == NULL)
 				fatal_exit("getcwd: %s", strerror(errno));
-			strncat(buf, "/", sizeof(buf));
+			strncat(buf, "/", sizeof(buf)-strlen(buf)-1);
 		}
-		strncat(buf, fname, sizeof(buf));
+		strncat(buf, fname, sizeof(buf)-strlen(buf)-1);
 		if(strncmp(buf, cfg->chrootdir, strlen(cfg->chrootdir)) != 0)
 			fatal_exit("config file %s is not inside chroot %s",
 				buf, cfg->chrootdir);