How to initialize a union?

Question

If it's a struct then it can be done

*p = {var1, var2..};

But seems this doesn't work with union:

union Ptrlist
{
        Ptrlist *next;
            State *s;
};

Ptrlist *l;
l = allocate_space();
*l = {NULL};

Only to get:

expected expression before ‘{’ token

Answer 1

In C99, you can use a designated union initializer:

union {
      char birthday[9];
      int age;
      float weight;
      } people = { .age = 14 };

In C++, unions can have constructors.

In C89, you have to do it explicitly.

typedef union {
  int x;
  float y;
  void *z;
} thing_t;

thing_t foo;
foo.x = 2;

By the way, are you aware that in C unions, all the members share the same memory space?

int main () 
{
   thing_t foo;
   printf("x: %p  y: %p  z: %p\n",
     &foo.x, &foo.y, &foo.z );
   return 0;
}

output:

x: 0xbfbefebc y: 0xbfbefebc z: 0xbfbefebc