[Tsp-devel] Use of opaque types

[Robert de Vries]

I see that the developers have used opaque types to hide the implementation of the structures from the end users.
A typical example is this:

/** Opaque handle type for provider */
typedef  void* TSP_provider_t;

This approach however has to disadvantages:
1. The type checking of the compiler is disabled, you can basically put any pointer as an argument to a function taking a TSP_provider_t as an argument
2. Debugging is made more complicated than necessary, when you try to do print *provider in gdb, it says "$20 = (TSP_provider_t) 0x8d52990" instead of

$21 = {server = 0x8d50128, server_info = {info = "rpc://localhost/StubbedServer:1", '\0' <repeats 225 times>},
  channel_id = 3, information = {version_id = 0, channel_id = 0, provider_timeout = 0, provider_group_number = 0,
    symbols = {TSP_sample_symbol_info_list_t_len = 1013, TSP_sample_symbol_info_list_t_val = 0x8d6e6b8},
    base_frequency = 100, max_period = 100000, max_client_number = 100, current_client_number = 4,
    status = TSP_STATUS_OK}, extended_informations = {TSP_sample_symbol_extended_info_list_t_len = 0,
    TSP_sample_symbol_extended_info_list_t_val = 0x0}, requested_sym = {TSP_sample_symbol_info_list_t_len = 1,
    TSP_sample_symbol_info_list_t_val = 0x8d7c500}, groups = 0x8d7c580, receiver = 0x0, sample_fifo = 0x0,
  thread_receiver = 0, data_link_broken = 0}

which is more useful.

Now you may ask, how do I do this?
As follows:
In the public header you make a forward declaration:

typedef struct TSP_otsp_t TSP_provider_t;

In an internal header or in the source code you provide the real struct TSP_otsp_t.
You may note that I did not put a * in the type definition above. This is on purpose.
If you want to do a prototype as follows:
int tsp_func(const TSP_provider_t *prov, ...);
You tell the compiler that tsp_func is not going to touch prov. However if you made a typedef that includes the *, the only thing you can say is that the pointer is const and that we already know.

The problem I currently see as that with the current scheme of opaque types you not only hide the type, but also the type information, and that is not so good.

     Robert