diff --git a/lib/xdrmappings/GwXDRMappings.cpp b/lib/xdrmappings/GwXDRMappings.cpp
index a0a70bb..1207693 100644
--- a/lib/xdrmappings/GwXDRMappings.cpp
+++ b/lib/xdrmappings/GwXDRMappings.cpp
@@ -374,7 +374,7 @@ GwXDRFoundMapping GwXDRMappings::getMapping(GwXDRCategory category,int selector,
     }
     return rt;
 }
-#define MAX_UNKNOWN 200
+
 bool GwXDRMappings::addUnknown(GwXDRCategory category,int selector,int field,int instance){
     if (unknown.size() >= 200) return false;
     unsigned long uk=((int)category) &0x7f;
@@ -388,16 +388,14 @@ bool GwXDRMappings::addUnknown(GwXDRCategory category,int selector,int field,int
     return true;
 }
 
-char * GwXDRMappings::getUnMapped(){
-    const int ESIZE=13;
-    int sz=(unknown.size()+1)*ESIZE;
-    char *rt=new char[sz];
-    *rt=0;
-    char *ptr=rt;
+const char * GwXDRMappings::getUnMapped(){
+    if (unknowAsString == NULL) unknowAsString=new char[(MAX_MAPPINGS+1)*ESIZE];
+    *unknowAsString=0;
+    char *ptr=unknowAsString;
     for (auto it=unknown.begin();it!=unknown.end();it++){
         snprintf(ptr,ESIZE-1,"%lu,",*it);
         *(ptr+ESIZE-1)=0;
         while (*ptr != 0) ptr++;
     }
-    return rt;
+    return unknowAsString;
 }
\ No newline at end of file
diff --git a/lib/xdrmappings/GwXDRMappings.h b/lib/xdrmappings/GwXDRMappings.h
index 3c7e7f0..0cb04b6 100644
--- a/lib/xdrmappings/GwXDRMappings.h
+++ b/lib/xdrmappings/GwXDRMappings.h
@@ -164,13 +164,18 @@ class GwXDRFoundMapping{
         }
 };
 
+//the class GwXDRMappings is not intended to be deleted
+//the deletion will leave memory leaks!
 class GwXDRMappings{
+    static const int MAX_UNKNOWN=200;
+    static const int ESIZE=13;
     private:
      GwLog *logger;
      GwConfigHandler *config;
      GwXDRMapping::N138Map n183Map;
      GwXDRMapping::N2KMap n2kMap;
      std::unordered_set<unsigned long> unknown;
+     char *unknowAsString=NULL;
      GwXDRFoundMapping selectMapping(GwXDRMapping::MappingList *list,int instance,const char * key);
      bool addUnknown(GwXDRCategory category,int selector,int field=0,int instance=-1);
     public:
@@ -180,8 +185,7 @@ class GwXDRMappings{
         //the returned mapping will exactly contain one mapping def
         GwXDRFoundMapping getMapping(String xName,String xType,String xUnit);
         GwXDRFoundMapping getMapping(GwXDRCategory category,int selector,int field=0,int instance=-1);
-        //returns a newly created buffer - user must destroy!
-        char * getUnMapped();
+        const char * getUnMapped();
 
 };