ENTITY* entSkycube =
{
   type = "skycube+6.tga";
   flags = UNLIT;
   flags2 = SKY | CUBE | SHOW;
}

int main ()
{
   //...

   level_load("test_0001.wmb");
   
   //...
   
   int ents = 0;
   ENTITY* ent = ptr_first(level_ent);
   while (ent)
   {
      ents++;
      ent = ent.link.next;
   }

   printf("ents = %d", ents);
   //out: "ents = 0"
}

...
   int ents = 0;
   ENTITY* seed = ent_createlocal(CUBE_MDL, nullvector, NULL);
   ENTITY* ent = ptr_first(seed);
   while (ent)
   {
      if(ent != seed)  //dont count the seed-entity
      {   ents++;   }
      ent = ent.link.next;
   }

   ent_remove(seed);
   printf("ents = %d", ents);
   //out: "ents = 0"

...

int ents = 0;
ENTITY* seed = ent_createlayer("", SKY, 0);
ENTITY* ent = ptr_first(seed);
while (ent)
{
   if (ent != seed)
   {
      error(ent->type); // "skycube+6.tga"
      ents++;
   }

   ent = ent.link.next;
}

if (seed)
   ptr_remove(seed);

printf("sky entities = %d", ents);