diff --git a/src/sm_genid.c b/src/sm_genid.c
index 9a52e1d..3a7d1e4 100644
--- a/src/sm_genid.c
+++ b/src/sm_genid.c
@@ -64,7 +64,9 @@ in this Software without prior written authorization from The Open Group.
 # include <X11/Xthreads.h>
 #endif
 #include <stdio.h>
-#include <unistd.h>
+#ifdef HAVE_UNISTD_H
+ #include <unistd.h>
+#endif
 
 #include <time.h>
 #define Time_t time_t