diff --git a/libbb/libbb.h b/libbb/libbb.h
index 3cf932d..66acc22 100644
--- a/libbb/libbb.h
+++ b/libbb/libbb.h
@@ -212,6 +212,8 @@
 char *concat_path_file(const char *path, const char *filename);
 char *last_char_is(const char *s, int c);
 
+extern long arith (const char *startbuf);
+
 typedef struct file_headers_s {
 	char *name;
 	char *link_name;