diff -Naur a/jam.h b/jam.h
--- a/jam.h	2007-05-13 20:58:24.000000000 +0000
+++ b/jam.h	2024-11-11 10:32:24.866235316 +0000
@@ -380,6 +380,7 @@
 # include <ctype.h>
 # include <signal.h>
 # include <string.h>
+# include <unistd.h>
 # include <time.h>
 
 # ifndef OS_QNX