diff --git a/ioctl.h b/ioctl.h
index 2cf9c73e..7f7e1d67 100644
--- a/ioctl.h
+++ b/ioctl.h
@@ -506,6 +506,7 @@ static inline char *btrfs_err_str(enum btrfs_err_code err_code)
 #define BTRFS_IOC_SCAN_DEV _IOW(BTRFS_IOCTL_MAGIC, 4, \
 				   struct btrfs_ioctl_vol_args)
 
+/* With a @src_length of zero, the range from @src_offset->EOF is cloned! */
 struct btrfs_ioctl_clone_range_args {
 	__s64 src_fd;
 	__u64 src_offset, src_length;