Make XT_REQUEST_NULL a literal constant.

* Dereferencing an external variable is unneeded and error-prone,
  because it can be overwritten.

src/xt/xt_request.h

@@ -50,7 +50,7 @@
  */
 
 typedef struct Xt_request_ *Xt_request;
-extern Xt_request XT_REQUEST_NULL;
+#define XT_REQUEST_NULL ((Xt_request)NULL)
 
 /**
  * Waits for the completion of a request. Afterwards it is deallocated and

src/xt_request.c

@@ -54,8 +54,6 @@
 #include "xt/xt_request.h"
 #include "xt_request_internal.h"
 
-Xt_request XT_REQUEST_NULL = (Xt_request)NULL;
-
 void xt_request_wait(Xt_request *request) {
   assert(request);
   if (*request != XT_REQUEST_NULL) {

src/xt_request_f.f90

@@ -78,6 +78,8 @@ MODULE xt_requests
 
   END INTERFACE
 
+  TYPE(xt_request), PARAMETER, PUBLIC :: xt_request_null = xt_request(c_null_ptr)
+
   INTERFACE xt_is_null
     MODULE PROCEDURE xt_request_is_null
   END INTERFACE xt_is_null

src/yaxt.f90

@@ -101,7 +101,7 @@ MODULE yaxt
   USE xt_redist_real_sp, ONLY: xt_redist_s_exchange, xt_redist_a_exchange
   USE xt_redist_real_dp, ONLY: xt_redist_s_exchange, xt_redist_a_exchange
   USE xt_requests, ONLY: xt_request, xt_request_wait, xt_request_test, &
-       xt_is_null
+       xt_is_null, xt_request_null
   USE xt_mpi, ONLY: xt_mpi_comm_mark_exclusive
 
   USE iso_c_binding, ONLY: c_int, c_ptr, c_null_ptr, c_loc
@@ -152,7 +152,8 @@ MODULE yaxt
        xt_mpi_comm_mark_exclusive, &
        xt_is_null, &
        OPERATOR(==), OPERATOR(/=), char, &
-       xt_request, xt_request_wait, xt_request_test, xt_redist_a_exchange1, &
+       xt_request, xt_request_null, &
+       xt_request_wait, xt_request_test, xt_redist_a_exchange1, &
        xt_redist_a_exchange
 
   INTERFACE OPERATOR(==)