diff --git a/kernel/defs.h b/kernel/defs.h
index 4eedd89..4b9bbc0 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -62,13 +62,13 @@ void            ramdiskrw(struct buf*);
 // kalloc.c
 void*           kalloc(void);
 void            kfree(void *);
-void            kinit();
+void            kinit(void);
 
 // log.c
 void            initlog(int, struct superblock*);
 void            log_write(struct buf*);
-void            begin_op();
-void            end_op();
+void            begin_op(void);
+void            end_op(void);
 
 // pipe.c
 int             pipealloc(struct file**, struct file**);
@@ -181,7 +181,7 @@ void            plic_complete(int);
 // virtio_disk.c
 void            virtio_disk_init(void);
 void            virtio_disk_rw(struct buf *, int);
-void            virtio_disk_intr();
+void            virtio_disk_intr(void);
 
 // number of elements in fixed-size array
 #define NELEM(x) (sizeof(x)/sizeof((x)[0]))