diff -r -u grpc-old/src/core/tsi/alts/crypt/gsec.h grpc/src/core/tsi/alts/crypt/gsec.h
--- a/src/core/tsi/alts/crypt/gsec.h
+++ b/src/core/tsi/alts/crypt/gsec.h
@@ -28,8 +28,10 @@
 #include <grpc/event_engine/port.h>
 #include <grpc/grpc.h>

-#ifndef _STRUCT_IOVEC
+#ifndef _STRUCT_IOVEC && !defined(__DEFINED_struct_iovec)
 #if !defined(GRPC_EVENT_ENGINE_POSIX)
+#define _STRUCT_IOVEC
+#define __DEFINED_struct_iovec
 struct iovec {
   void* iov_base;
   size_t iov_len;