tor/doc/HACKING/CodeStructure.md

TODO: revise this to talk about how things are, rather than how things have changed.

For quite a while now, the program tor has been built from source code in just two directories: src/common and src/or.

This has become more-or-less untenable, for a few reasons -- most notably of which is that it has led our code to become more spaghetti-ish than I can endorse with a clean conscience.

So to fix that, we've gone and done a huge code movement in our git master branch, which will land in a release once Tor 0.3.5.1-alpha is out.

Here's what we did:

• src/common has been turned into a set of static libraries. These all live in the src/lib/* directories. The dependencies between these libraries should have no cycles. The libraries are:

  • arch -- Headers to handle architectural differences
  • cc -- headers to handle differences among compilers
  • compress -- wraps zlib, zstd, lzma
  • container -- high-level container types
  • crypt_ops -- Cryptographic operations. Planning to split this into a higher and lower level library
  • ctime -- Operations that need to run in constant-time. (Properly, data-invariant time)
  • defs -- miscelaneous definitions needed throughout Tor.
  • encoding -- transforming one data type into another, and various data types into strings.
  • err -- lowest-level error handling, in cases where we can't use the logs because something that the logging system needs has broken.
  • evloop -- Generic event-loop handling logic
  • fdio -- Low-level IO wrapper functions for file descriptors.
  • fs -- Operations on the filesystem
  • intmath -- low-level integer math and misc bit-twiddling hacks
  • lock -- low-level locking code
  • log -- Tor's logging module. This library sits roughly halfway up the library dependency diagram, since everything it depends on has to be carefully crafted to not log.
  • malloc -- Low-level wrappers for the platform memory allocation functions.
  • math -- Higher-level mathematical functions, and floating-point math
  • memarea -- An arena allocator
  • meminfo -- Functions for querying the current process's memory status and resources
  • net -- Networking compatibility and convenience code
  • osinfo -- Querying information about the operating system
  • process -- Launching and querying the status of other processes
  • sandbox -- Backend for the linux seccomp2 sandbox
  • smartlist_core -- The lowest-level of the smartlist_t data type. Separated from the rest of the containers library because the logging subsystem depends on it.
  • string -- Compatibility and convenience functions for manipulating C strings.
  • term -- Terminal-related functions (currently limited to a getpass function).
  • testsupport -- Macros for mocking, unit tests, etc.
  • thread -- Higher-level thread compatibility code
  • time -- Higher-level time management code, including format conversions and monotonic time
  • tls -- Our wrapper around our TLS library
  • trace -- Formerly src/trace -- a generic event tracing API
  • wallclock -- Low-level time code, used by the log module.

• To ensure that the dependency graph in src/common remains under control, there is a tool that you can run called make check-includes. It verifies that each module in Tor only includes the headers that it is permitted to include, using a per-directory .may_include file.

• The src/or/or.h header has been split into numerous smaller headers. Notably, many important structures are now declared in a header called foo_st.h, where "foo" is the name of the structure.

• The src/or directory, which had most of Tor's code, had been split up into several directories. This is still a work in progress: This code has not itself been refactored, and its dependency graph is still a tangled web. I hope we'll be working on that over the coming releases, but it will take a while to do.

  • The new top-level source directories are:

    • src/core -- Code necessary to actually perform or use onion routing.
    • src/feature -- Code used only by some onion routing configurations, or only for a special purpose.
    • src/app -- Top-level code to run, invoke, and configure the lower-level code

  • The new second-level source directories are:

    • src/core/crypto -- High-level cryptographic protocols used in Tor
    • src/core/mainloop -- Tor's event loop, connection-handling, and traffic-routing code.
    • src/core/or -- Parts related to handling onion routing itself
    • src/core/proto -- support for encoding and decoding different wire protocols
    • src/feature/api -- Support for making Tor embeddable
    • src/feature/client -- Functionality which only Tor clients need
    • src/feature/control -- Controller implementation
    • src/feature/dirauth -- Directory authority
    • src/feature/dircache -- Directory cache
    • src/feature/dirclient -- Directory client
    • src/feature/dircommon -- Shared code between the other directory modules
    • src/feature/hibernate -- Hibernating when Tor is out of bandwidth or shutting down
    • src/feature/hs -- v3 onion service implementation
    • src/feature/hs_common -- shared code between both onion service implementations
    • src/feature/nodelist -- storing and accessing the list of relays on the network.
    • src/feature/relay -- code that only relay servers and exit servers need.
    • src/feature/rend -- v2 onion service implementation
    • src/feature/stats -- statistics and history
    • src/app/config -- configuration and state for Tor
    • src/app/main -- Top-level functions to invoke the rest or Tor.

• The tor executable is now built in src/app/tor rather than src/or/tor.

• There are more static libraries than before that you need to build into your application if you want to embed Tor. Rather than maintaining this list yourself, I recommend that you run make show-libs to have Tor emit a list of what you need to link.