How to pass a function as an argument, with arguments?

Question

So, I'm playing with callbacks in GNUTLS.

I want to pass my function, handleSNI to a function, but I also want to pass on some other important arguments as well(to handleSNI when it is called).

I could just do them outside the functions in global variables, but that isn't thread safe.

Is it possible? If not, how can one accomplish this?

Are you using a class? Because if you are not, it would be easier to implement what you are talking about with a class. — Gabriel, Aug 16 '15 at 00:46

score 1 · Accepted Answer · answered Aug 15 '15 at 23:58

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GnuTLS supports user defined arguments in callbacks. For the push and pull function you set them via gnutls_transport_set_ptr. You pass a pointer to a struct containing your data and use that in your callback.

On a more general level the answer is: If your library doesn't support user defined parameters, you are out of luck. In C there is no way to cheat an extra parameter in. If you ever find such a library, do your self a favor and don't use it. And if there is no alternative you could use threadsafe containers or thread local storage. But it is really ugly.

answered Aug 15 '15 at 23:58

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Doesn't set_ptr equivilant to setting the sockfd in set_int? – JavaProphet Aug 16 '15 at 00:17
A __thread prefixed global solved the issue nicely. – JavaProphet Aug 16 '15 at 01:25

How to pass a function as an argument, with arguments?

1 Answers1