The code, something that i dont understand

[DÔ Phan-Cam-Thach]

I have red the code, there are something i dont understand.
In this line:
void add_timeout PROTO ((TIME, void  (*) PROTO ((void *)), void *, tvref_t, tvunref_t));
what does it mean, PROTO?
And someone knows what exactly does the function add_timeout do? I know it adds a timeout, but how and what are in the code exactly? 
Thanks in advance for your helps.


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