Question

Now that I found a way to statically initialize my array of items, I need a more complexe structure and instead of a char* as a value, I need a struct (named atom_s).

typdef struct atom_s {
  const char *val;
  const char *filter;
} atom_t;

struct
{
  const char *key;
  const atom_t **values;
} key_to_values[] =
{
  { .key = "foo", .values = (const atom_t *[]) { NULL } },
  { .key = "bar", .values = (const atom_t *[]) { { .val = "foo", .filter = "bar" }, NULL } },
};

The problem is: I don’t know how to initialize an atom_s inside the array declaration above or if it is even possible.
The second line of the array (with key = “bar”) does not compile:

warning: braces around scalar initializer  
warning: (near initialization for '(anonymous)[0]')
error: field name not in record or union initializer

Answer 1

{ 
   .key = "bar", 
   .values = (const atom_t *[]) {
       (const atom_t []) {
           { .val = "foo", .filter = "bar" }
           // how is the user going to know this array has one element?
       },
       NULL 
   }
},

Each element of the array pointed to by values, points to an array of atom_t (except the last, which is null). You might also need some way to terminate each of those inner arrays, unless they’re always the same length.