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I started this repository a while ago to work on documentation for Tor's internals. It needs substantial revision, but first, let's get it copied into Tor's repository. These files are copied, "warts and all", from the tor-guts.git repo, commit de1e34259178b09861c0dea319c760fa80d0099a. Part of 31819.
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OS compatibility functions
We've got a bunch of functions to wrap differences between various operating systems where we run.
The filesystem
We wrap the most important filesystem functions with load-file, save-file, and map-file abstractions declared in util.c or compat.c. If you're messing about with file descriptors yourself, you might be doing things wrong. Most of the time, write_str_to_file() and read_str_from_file() are all you need.
Use the check_private_directory() function to create or verify the presence of directories, and tor_listdir() to list the files in a directory.
Those modules also have functions for manipulating paths a bit.
Networking
Nearly all the world is on a Berkeley sockets API, except for windows, whose version of the Berkeley API was corrupted by late-90s insistence on backward compatibility with the sort-of-berkeley-sort-of-not add-on thing that was WinSocks.
What's more, everybody who implemented sockets realized that select() wasn't a very good way to do nonblocking IO... and then the various implementations all decided to so something different.
You can forget about most of these differences, fortunately: We use libevent to hide most of the differences between the various networking backends, and we add a few of our own functions to hide the differences that Libevent doesn't.
To create a network connection, the right level of abstraction to look at is probably the connection_t system in connection.c. Most of the lower level work has already been done for you. If you need to instantiate something that doesn't fit well with connection_t, you should see whether you can instantiate it with connection_t anyway -- or you might need to refactor connection.c a little.
Whenever possible, represent network addresses as tor_addr_t.
Process launch and monitoring
Launching and/or monitoring a process is tricky business. You can use the mechanisms in procmon.c and tor_spawn_background(), but they're both a bit wonky. A refactoring would not be out of order.