diff --git a/winsup/cygwin/ntdll.h b/winsup/cygwin/ntdll.h
index b251055f6..64babefc3 100644
--- a/winsup/cygwin/ntdll.h
+++ b/winsup/cygwin/ntdll.h
@@ -157,6 +157,7 @@ extern GUID __cygwin_socket_guid;
 #define FILE_VC_VALID_MASK              0x000003ff
 
 /* IOCTL code to impersonate client of named pipe. */
+
 #define FSCTL_PIPE_DISCONNECT	CTL_CODE(FILE_DEVICE_NAMED_PIPE, 1, \
 					 METHOD_BUFFERED, FILE_ANY_ACCESS)
 #define FSCTL_PIPE_LISTEN	CTL_CODE(FILE_DEVICE_NAMED_PIPE, 2, \
@@ -165,6 +166,8 @@ extern GUID __cygwin_socket_guid;
 					 METHOD_BUFFERED, FILE_ANY_ACCESS)
 #define FSCTL_PIPE_IMPERSONATE	CTL_CODE(FILE_DEVICE_NAMED_PIPE, 7, \
 					 METHOD_BUFFERED, FILE_ANY_ACCESS)
+#define FSCTL_PIPE_FLUSH	CTL_CODE(FILE_DEVICE_NAMED_PIPE, 16, \
+					 METHOD_BUFFERED, FILE_WRITE_DATA)
 
 typedef enum _FILE_INFORMATION_CLASS
 {