Add a helper function to get a file from its descriptor (FD), w/ E

src/userprog/syscall.c

@@ -281,6 +281,22 @@ fd_less (const struct hash_elem *a_, const struct hash_elem *b_,
   return a->fd < b->fd;
 }
 
+/* Gets a file from its descriptor (FD number). If there is no file with the fd
+   FD it returns NULL. */
+static struct file *
+fd_get_file (int fd)
+{
+  /* We have to set up a fake open_file in order to be able to search the hash
+     table. See hash.h. */
+  struct open_file *fake_file_info;
+  fake_file_info->fd = fd;
+
+  struct file *file
+    = hash_find (thread_current ()->open_files, fake_file_info->elem);
+
+  return file;
+}
+
 /* Validates if a block of memory starting at PTR and of size SIZE bytes is
    fully contained within user virtual memory. Kills the thread (by calling
    thread_exit) if the memory is invalid. Otherwise, returns the PTR given.