diff --git a/include/picrin/port.h b/include/picrin/port.h
index 0b89e373..857d2deb 100644
--- a/include/picrin/port.h
+++ b/include/picrin/port.h
@@ -16,6 +16,9 @@ enum pic_port_status {
 };
 
 typedef struct {
+  char *buf;
+  int mode;
+  int bufsiz;
   struct {
     void *cookie;
     int (*read)(void *, char *, int);
@@ -41,6 +44,7 @@ struct pic_port *pic_stdin(pic_state *);
 struct pic_port *pic_stdout(pic_state *);
 struct pic_port *pic_stderr(pic_state *);
 
+int pic_setvbuf(pic_file *, char *, int, size_t);
 int pic_fflush(pic_file *);
 
 pic_file *pic_funopen(void *cookie, int (*read)(void *, char *, int), int (*write)(void *, const char *, int), fpos_t (*seek)(void *, fpos_t, int), int (*close)(void *));
diff --git a/src/port.c b/src/port.c
index 11e835e7..2c7fd2c7 100644
--- a/src/port.c
+++ b/src/port.c
@@ -37,6 +37,32 @@ pic_stdout(pic_state *pic)
   return pic_port_ptr(pic_apply(pic, proc, pic_nil_value()));
 }
 
+int
+pic_setvbuf(pic_file *file, char *buf, int mode, size_t bufsiz)
+{
+  /* FIXME: free old buf */
+
+  file->mode = mode;
+  if (buf) {
+    file->buf = buf;
+    file->bufsiz = bufsiz;
+  }
+  else {
+    if (mode == _IONBF) {
+      file->buf = NULL;
+      file->bufsiz = 0;
+    }
+    else {
+      assert(bufsiz == 0);
+      file->buf = (char *)malloc(BUFSIZ);
+      file->bufsiz = BUFSIZ;
+    }
+  }
+  file->s = file->c = file->buf;
+  file->e = file->buf + file->bufsiz;
+  return 0;
+}
+
 pic_file *
 pic_funopen(void *cookie,
             int (*read)(void *, char *, int),