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diff --git a/src/lib/subsys/lib_subsys.md b/src/lib/subsys/lib_subsys.md new file mode 100644 index 0000000000..764d25d1b6 --- /dev/null +++ b/src/lib/subsys/lib_subsys.md @@ -0,0 +1,32 @@ +@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. + |