diff --git a/src/drv_maildir.c b/src/drv_maildir.c
index b62848e..46b4cb1 100644
--- a/src/drv_maildir.c
+++ b/src/drv_maildir.c
@@ -43,7 +43,7 @@
 # define LEGACY_FLOCK 1
 #endif
 
-#ifndef _POSIX_SYNCHRONIZED_IO
+#if !defined(_POSIX_SYNCHRONIZED_IO) || _POSIX_SYNCHRONIZED_IO <= 0
 # define fdatasync fsync
 #endif
 
diff --git a/src/sync.c b/src/sync.c
index 687911d..851b2db 100644
--- a/src/sync.c
+++ b/src/sync.c
@@ -35,7 +35,7 @@
 #include <errno.h>
 #include <sys/stat.h>
 
-#ifndef _POSIX_SYNCHRONIZED_IO
+#if !defined(_POSIX_SYNCHRONIZED_IO) || _POSIX_SYNCHRONIZED_IO <= 0
 # define fdatasync fsync
 #endif