How do I pass a struct into the browse_callback and then further down to resolve_callback in avahi?

Question

I want to populate a widget whenever a service is successfully resolved in avahi. Since most of the information required to populate is available inside resolve_callback(), I need this widget to be present inside it. But resolve_callback() being a callback does not take any arguments that I can modify. It is automatically called by avahi_service_resolver_new().

static void resolve_callback(
    AvahiServiceResolver *r,
    AVAHI_GCC_UNUSED AvahiIfIndex interface,
    AVAHI_GCC_UNUSED AvahiProtocol protocol,
    AvahiResolverEvent event,
    const char *name,
    const char *type,
    const char *domain,
    const char *host_name,
    const AvahiAddress *address,
    uint16_t port,
    AvahiStringList *txt,
    AvahiLookupResultFlags flags,
    PpDnsWindow *self)
{
    assert(r);
    /* Called whenever a service has been resolved successfully or timed out */
    switch (event)
    {
    case AVAHI_RESOLVER_FAILURE:
         fprintf(stderr, "(Resolver) Failed to resolve service '%s' of type '%s' in domain '%s': %s\n", name, type, domain, avahi_strerror(avahi_client_errno(avahi_service_resolver_get_client(r))));
         break;
    case AVAHI_RESOLVER_FOUND:
    {
        // here I'll be using the self struct

        char a[AVAHI_ADDRESS_STR_MAX], *t;
        fprintf(stderr, "Service '%s' of type '%s' in domain '%s':\n", name, type, domain);
        avahi_address_snprint(a, sizeof(a), address);
        t = avahi_string_list_to_string(txt);
        fprintf(stderr,
                "\t%s:%u (%s)\n"
                "\tTXT=%s\n"
                "\tcookie is %u\n"
                "\tis_local: %i\n"
                "\tour_own: %i\n"
                "\twide_area: %i\n"
                "\tmulticast: %i\n"
                "\tcached: %i\n",
                host_name, port, a,
                t,
                avahi_string_list_get_service_cookie(txt),
                !!(flags & AVAHI_LOOKUP_RESULT_LOCAL),
                !!(flags & AVAHI_LOOKUP_RESULT_OUR_OWN),
                !!(flags & AVAHI_LOOKUP_RESULT_WIDE_AREA),
                !!(flags & AVAHI_LOOKUP_RESULT_MULTICAST),
                !!(flags & AVAHI_LOOKUP_RESULT_CACHED));
        avahi_free(t);
    }
    }
    avahi_service_resolver_free(r);
}

static void browse_callback(
    AvahiServiceBrowser *b,
    AvahiIfIndex interface,
    AvahiProtocol protocol,
    AvahiBrowserEvent event,
    const char *name,
    const char *type,
    const char *domain,
    AVAHI_GCC_UNUSED AvahiLookupResultFlags flags,
    PpDnsWindow *self)
{
    AvahiClient *c = self->client;
    assert(b);
    /* Called whenever a new services becomes available on the LAN or is removed from the LAN */
    switch (event)
    {
    case AVAHI_BROWSER_FAILURE:
        fprintf(stderr, "(Browser) %s\n", avahi_strerror(avahi_client_errno(avahi_service_browser_get_client(b))));
        return;
    case AVAHI_BROWSER_NEW:
        fprintf(stderr, "(Browser) NEW: service '%s' of type '%s' in domain '%s'\n", name, type, domain);
        /* We ignore the returned resolver object. In the callback
               function we free it. If the server is terminated before
               the callback function is called the server will free
               the resolver for us. */
        if (!(avahi_service_resolver_new(c, interface, protocol, name, type, domain, AVAHI_PROTO_UNSPEC, 0, resolve_callback, self)))
            fprintf(stderr, "Failed to resolve service '%s': %s\n", name, avahi_strerror(avahi_client_errno(c)));
        break;
    case AVAHI_BROWSER_REMOVE:
        fprintf(stderr, "(Browser) REMOVE: service '%s' of type '%s' in domain '%s'\n", name, type, domain);
        break;
    case AVAHI_BROWSER_ALL_FOR_NOW:
    case AVAHI_BROWSER_CACHE_EXHAUSTED:
        fprintf(stderr, "(Browser) %s\n", event == AVAHI_BROWSER_CACHE_EXHAUSTED ? "CACHE_EXHAUSTED" : "ALL_FOR_NOW");
        break;
    }
}

I tried modifying AVAHI_GCC_UNUSED void* userdata to PpDnsWindow* self in this function definition and then passed PpDnsWindow down these functions like so-

/* Create the service browser */
  if (!(sb = avahi_service_browser_new(self->client, AVAHI_IF_UNSPEC, AVAHI_PROTO_UNSPEC, "_ipp._tcp", NULL, 0, browse_callback, self)))
  {
      fprintf(stderr, "Failed to create service browser: %s\n", avahi_strerror(avahi_client_errno(self->client)));
      destroy(self);
  }

But this leades to resolve failure with AVAHI_RESOLVER_FAILURE.

I don't want to make the widget global since globals are generally discouraged. Is there some way I can achieve this?

Answer 1

I was also supposed to add AVAHI_GCC_UNUSED before PpDnsWindow *self when changing the function signature for resolve_callback() as follows. But this is not required for browse_callback().

static void resolve_callback(
    AvahiServiceResolver *r,
    AVAHI_GCC_UNUSED AvahiIfIndex interface,
    AVAHI_GCC_UNUSED AvahiProtocol protocol,
    AvahiResolverEvent event,
    const char *name,
    const char *type,
    const char *domain,
    const char *host_name,
    const AvahiAddress *address,
    uint16_t port,
    AvahiStringList *txt,
    AvahiLookupResultFlags flags,
    AVAHI_GCC_UNUSED PpDnsWindow *self)

I looked up AVAHI_GCC_UNUSED online but could'nt find anything substantial. I would appreciate if some explained it's significance.