title: Assigning the value of an union containing structures to NULL in c programming
body: <p>I have a union and 2 structures in the following format and I need to set one of them to NULL.
  For example m = NULL or t = NULL;    </p>
  
  <pre><code> typedef struct
   {
      char *population;
     char *area;
  
   } metropolitan;
  
  
   typedef struct
   {
      char *airport;
      char *type;
  
   } tourist;
  
  
   typedef union
   {
       tourist t;
       metropolitan m;
   } ex;
  </code></pre>