diff --git a/lib/ftruncate.c b/lib/ftruncate.c
index 4f6aaa993..58cd2bba9 100644
--- a/lib/ftruncate.c
+++ b/lib/ftruncate.c
@@ -32,7 +32,9 @@ ftruncate (fd, length)
 
 #include <sys/stat.h>
 #include <errno.h>
+#ifdef HAVE_UNISTD_H
 #include <unistd.h>
+#endif
 
 int
 ftruncate (fd, length)