diff --git a/Modules/Setup.in b/Modules/Setup.in
index ae9352b..994c687 100644
--- a/Modules/Setup.in
+++ b/Modules/Setup.in
@@ -150,10 +150,13 @@
 pwd pwdmodule.c		# pwd(3) 
 grp grpmodule.c		# grp(3)
 errno errnomodule.c	# posix (UNIX) errno values
-mmap mmapmodule.c       # Memory-mapped files (also works on Win32)
 select selectmodule.c	# select(2); not on ancient System V
 _socket socketmodule.c	# socket(2)
 
+# Memory-mapped files (also works on Win32).
+# Some platforms require -lucb.
+mmap mmapmodule.c # -lucb
+
 # Socket module compiled with SSL support; you must edit the SSL variable:
 #SSL=/usr/local/ssl
 #_socket socketmodule.c \