--- squashfs-tools/mksquashfs.h
+++ squashfs-tools/mksquashfs.h
@@ -132,7 +132,7 @@ struct append_file {
 #define BLOCK_OFFSET 2
 
 extern struct cache *reader_buffer, *fragment_buffer, *reserve_cache;
-struct cache *bwriter_buffer, *fwriter_buffer;
+extern struct cache *bwriter_buffer, *fwriter_buffer;
 extern struct queue *to_reader, *to_deflate, *to_writer, *from_writer,
 	*to_frag, *locked_fragment, *to_process_frag;
 extern struct append_file **file_mapping;