diff --git a/include/picrin/port.h b/include/picrin/port.h
index dd5b8ae0..4d12d6e7 100644
--- a/include/picrin/port.h
+++ b/include/picrin/port.h
@@ -15,11 +15,19 @@ enum pic_port_status {
   PIC_PORT_CLOSE,
 };
 
+#define PIC_UBUFSIZ 3
+
 typedef struct {
+  /* buffered IO */
   char *buf;
   int mode;
   int bufsiz;
   char *s, *c, *e;
+  /* ungetc buf */
+  char ub[PIC_UBUFSIZ];
+  int us;
+  int ur;
+  /* operators */
   struct {
     void *cookie;
     int (*read)(void *, char *, int);
diff --git a/src/port.c b/src/port.c
index b6f4845c..3a48c940 100644
--- a/src/port.c
+++ b/src/port.c
@@ -94,6 +94,8 @@ pic_funopen(void *cookie,
   file->buf = NULL;
   file->mode = _IONBF;
   file->bufsiz = 0;
+  file->us = PIC_UBUFSIZ;
+  file->ur = 0;
   /* set vtable */
   file->vtable.cookie = cookie;
   file->vtable.read = read;