C generic inheritance with low verbosity

Question

Having:

struct packet_sample_t {
   int common_id;
   int some;
   char data[];
   float foo;
}

Having the following function:

void dispatch_packet(void *packet);

My goal would be to parse the packet id and then call its handler, but I can't retrieve the struct variable common_id from void *.

I would like to create something like an interface in hight level languages, assuming that all my packets structures should have the variable common_id.
So I'm looking something that would work like below:

struct packet_base {
   int common_id;
}

void dispatch_packet(void *packet) {
   int common_id = ( (packet_base *)packet )->common_id;
   switch(common_id) {...}
}

void test() {
   packet_sample_t packet = {.common_id = 10, ...};
   dispatch_packet((void *) &packet); //this function should retrieve `common_id`

   packet_other_t other = {.common_id = 1};
   dispatch_packet((void *) &other); // again with another packet
}

Im not that familiar to C language and I dont really know how I could do this. But in simple words, I would like to be able to cast a packet to its packet_base, that are sharing both a common variable.

EDIT: more details in the example

Answer 1

Your technique is valid. There's a number of ways to do struct inheritance in C, and this is one of them. 21st Century C might be a good read for you as well as Object-Oriented Programming with ANSI C.

You have a problems with how you're declaring and using your structs and types. Let's look at this.

struct packet_base {
   int common_id;
};

This has the type struct packet_base. If you want to declare a pointer to this type you need to write struct packet_base *packet. If you want to cast a variable of this type it's (struct packet_base *)thing.

This is annoying, so you typically declare a type alias to the struct using typedef. The syntax is typedef <type> <alias>

typedef struct {
   int common_id;
} packet_base_t;

That says the type struct { int common_id; } is aliased to packet_base_t. Now you can use packet_base_t as the type. packet_base_t *packet declares a pointer and (packet_base_t *)thing casts.

With that fixed, plus some small errors, it works. See What is the difference between char array vs char pointer in C? for char *data vs char data[].

typedef struct {
   int common_id;
   int some;
   char *data;
   float foo;
} packet_sample_t;

typedef struct {
   int common_id;
} packet_base_t;

void dispatch_packet(void *arg) {
    // It's simpler to cast to a new variable once then to muck
    // up the code with casts.
    packet_base_t *packet = (packet_base_t *)arg;

    int common_id = packet->common_id;
    printf("id: %d\n", common_id);
}

Answer 2

You can use union to aggregate the data of different types in a single structure

struct packet1_t
{
   // Packet1 specific data
   ......
}

struct packet2_t
{
   // Packet2 specific data
   ......
}

struct packet_t
{
    int common_id;
    union
    {
        struct packet1_t packet1;
        struct packet2_t packet2;
        ......
    }
} packet;

Now, based on the ID you can pick the correct type from the union:

int do_something_with_packet(packet_t packet)
{
    switch (packet.common_id)
    {
        case 1:
            do_something_with_packet1(packet.packet1);
            break;
        case 2:
            do_something_with_packet2(packet.packet2);
            break;
        ..................
    }
    ....
}