diff --git a/src/lib/w32/posixfile.c b/src/lib/w32/posixfile.c
index 5bebc76d..ee6ec559 100644
--- a/src/lib/w32/posixfile.c
+++ b/src/lib/w32/posixfile.c
@@ -36,6 +36,10 @@
 #include <io.h>
 #include "unistd.h"
 
+/*
+ * POSIX equivalent for mkdir().
+ * Windows does not set the permission directly with mkdir().
+ */
 int
 posix_mkdir(const char *dirname, mode_t perm)
 {