diff --git a/src/threads/thread.h b/src/threads/thread.h
index bdfad35..2904022 100644
--- a/src/threads/thread.h
+++ b/src/threads/thread.h
@@ -137,6 +137,10 @@ struct thread
 
     struct hash pages;                  /* Table of open user pages. */
 
+    /* Memory mapped files for user virtual memory. */
+    struct hash mmap_files;             /* List of memory mapped files. */
+    unsigned int mmap_counter;          /* Counter for memory mapped files. */
+
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */