What does `static int (*const array[SIZE_ARRAY]…)(int a) = {[SOMETHING]=something}` mean in C?

Question

I'm having a hard time interpreting the following C code:

static int (* const net_client_init_fun[NET_CLIENT_DRIVER__MAX])(
    const Netdev *netdev,
    const char *name,
    NetClientState *peer, Error **errp) = {
        [NET_CLIENT_DRIVER_NIC]       = net_init_nic,
#ifdef CONFIG_SLIRP
        [NET_CLIENT_DRIVER_USER]      = net_init_slirp,
#endif
        [NET_CLIENT_DRIVER_TAP]       = net_init_tap,
        [NET_CLIENT_DRIVER_SOCKET]    = net_init_socket,
#ifdef CONFIG_VDE
        [NET_CLIENT_DRIVER_VDE]       = net_init_vde,
#endif
#ifdef CONFIG_NETMAP
        [NET_CLIENT_DRIVER_NETMAP]    = net_init_netmap,
#endif
#ifdef CONFIG_NET_BRIDGE
        [NET_CLIENT_DRIVER_BRIDGE]    = net_init_bridge,
#endif
        [NET_CLIENT_DRIVER_HUBPORT]   = net_init_hubport,
#ifdef CONFIG_VHOST_NET_USER
        [NET_CLIENT_DRIVER_VHOST_USER] = net_init_vhost_user,
#endif
#ifdef CONFIG_L2TPV3
        [NET_CLIENT_DRIVER_L2TPV3]    = net_init_l2tpv3,
#endif
};

this looks like a function pointer declaration, but then it does = {...} which I don't recognize. Is also does [] inside.

What this is exactly?

Answer 1

It might be easier to understand by using a type-alias:

// Define a type-alias for a pointer to a function
typedef int (*net_client_init_fun_type)(const Netdev *, const char *, NetClientState *, Error **);

// Define an array of pointers to functions
static net_client_init_fun_type net_client_init_fun[NET_CLIENT_DRIVER__MAX] = { ... }

It still requires knowledge about function-pointers and how they are declared/defined, but it will make the array declaration much easier to read.

---

The array initialization list uses designated array initialization .

When you have a line like:

[NET_CLIENT_DRIVER_NIC] = net_init_nic

in the array initialization list, it means that the index NET_CLIENT_DRIVER_NIC of the array will be initialized to net_init_nic .