diff --git a/revision.h b/revision.h
index f64e8ce7ff..1b045669ae 100644
--- a/revision.h
+++ b/revision.h
@@ -119,7 +119,7 @@ struct rev_info {
 void read_revisions_from_stdin(struct rev_info *revs);
 
 typedef void (*show_early_output_fn_t)(struct rev_info *, struct commit_list *);
-volatile show_early_output_fn_t show_early_output;
+extern volatile show_early_output_fn_t show_early_output;
 
 extern void init_revisions(struct rev_info *revs, const char *prefix);
 extern int setup_revisions(int argc, const char **argv, struct rev_info *revs, const char *def);