struct_node.c

// Common:
struct endpoint {
    struct node * node;
    pull_fn_pt pull;
    //const type_t * type;  // Maybe...
    //const char * name;    // Maybe...
};

// Type 1:
struct node {
    block_destroy_fn_pt destroy;

    object_t * state;

    size_t n_inputs;
    struct endpoint ** inputs;

    size_t n_outputs;
    struct endpoint outputs[];
};

node = malloc(sizeof(struct node) + n_outputs * sizeof(struct endpoint));
node->inputs = malloc(n_inputs * sizeof(struct endpoint *));

node->inputs[i]->node...
node->outputs[o].node...

// Type 2:

struct node {
    block_destroy_fn_pt destroy;

    object_t * state;

    size_t n_inputs;
    size_t n_outputs;
    union endpoint_or_ptr {
        struct endpoint * input;
        struct endpoint output;
    } endpoints[];
};

node = malloc(sizeof(struct node) + (n_inputs + n_outputs) * sizeof(union endpoint_or_ptr));

node->inputs[i].input->node...
node->outputs[n_inputs + o].output.node...