Nick Mathewson
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@dir /lib/subsys @brief lib/subsys: Types for declaring a "subsystem".
Subsystems in Tor
A subsystem is a module with support for initialization, shutdown, configuration, and so on.
Many parts of Tor can be initialized, cleaned up, and configured somewhat independently through a table-driven mechanism. Each such part is called a "subsystem".
To declare a subsystem, make a global const instance of the subsys_fns_t
type, filling in the function pointer fields that you require with ones
corresponding to your subsystem. Any function pointers left as "NULL" will
be a no-op. Each system must have a name and a "level", which corresponds to
the order in which it is initialized. (See app/main/subsystem_list.c for a
list of current subsystems and their levels.)
Then, insert your subsystem in the list in app/main/subsystem_list.c. It
will need to occupy a position corresponding to its level.
At this point, your subsystem will be handled like the others: it will get initialized at startup, torn down at exit, and so on.
Historical note: Not all of Tor's code is currently handled as
subsystems. As you work with older code, you may see some parts of the code
that are initialized from tor_init() or run_tor_main_loop() or
tor_run_main(); and torn down from tor_cleanup(). We aim to migrate
these to subsystems over time; please don't add any new code that follows
this pattern.