diff --git a/csync/src/csync_util.c b/csync/src/csync_util.c
index a79d925f84..b82005a16a 100644
--- a/csync/src/csync_util.c
+++ b/csync/src/csync_util.c
@@ -182,6 +182,10 @@ csync_vio_file_stat_t *csync_vio_convert_file_stat(csync_file_stat_t *st) {
 }
 
 bool (*csync_file_locked_or_open_ext) (const char*) = 0; // filled in by library user
+void set_csync_file_locked_or_open_ext(bool (*f) (const char*)) {
+    csync_file_locked_or_open_ext = f;
+}
+
 bool csync_file_locked_or_open( const char *dir, const char *fname) {
     char *tmp_uri = NULL;
     bool ret;