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113 lines
4.0 KiB
Markdown
113 lines
4.0 KiB
Markdown
# Modules in Tor #
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This document describes the build system and coding standards when writing a
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module in Tor.
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## What is a module? ##
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In the context of the tor code base, a module is a subsystem that we can
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selectively enable or disable, at `configure` time.
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Currently, tor has these modules:
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- Relay subsystem (relay)
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- Directory Authority subsystem (dirauth)
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dirauth is located in its own directory in `src/feature/dirauth/`.
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Relay is located in directories named `src/*/*relay` and `src/*/*dircache`,
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which are being progressively refactored and disabled.
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To disable a module, pass `--disable-module-{dirauth,relay}` at configure
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time. All modules are currently enabled by default.
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## Build System ##
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The changes to the build system are pretty straightforward.
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1. Locate in the `configure.ac` file this define: `m4_define(MODULES`. It
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contains a list (white-space separated) of the module in tor. Add yours to
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the list.
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2. Use the `AC_ARG_ENABLE([module-relay]` template for your new module. We
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use the "disable module" approach instead of enabling them one by one. So,
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by default, tor will build all the modules.
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This will define the `HAVE_MODULE_<name>` statement which can be used in
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the C code to conditionally compile things for your module. And the
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`BUILD_MODULE_<name>` is also defined for automake files (e.g: include.am).
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3. In the `src/core/include.am` file, locate the `MODULE_RELAY_SOURCES`
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value. You need to create your own `_SOURCES` variable for your module
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and then conditionally add the it to `LIBTOR_A_SOURCES` if you should
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build the module.
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It is then **very** important to add your SOURCES variable to
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`src_or_libtor_testing_a_SOURCES` so the tests can build it.
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Finally, your module will automatically be included in the
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`TOR_MODULES_ALL_ENABLED` variable which is used to build the unit tests.
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They always build everything in order to test everything.
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## Coding ##
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As mentioned above, a module should be isolated in its own directories,
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suffixed with the name of the module, in `src/*/`.
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There are couples of "rules" you want to follow:
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* Minimize as much as you can the number of entry points into your module.
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Less is always better but of course that doesn't work out for every use
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case. However, it is a good thing to always keep that in mind.
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* Do **not** use the `HAVE_MODULE_<name>` define outside of the module code
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base. Every entry point should have a second definition if the module is
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disabled. For instance:
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```
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#ifdef HAVE_MODULE_DIRAUTH
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int sr_init(int save_to_disk);
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#else /* HAVE_MODULE_DIRAUTH */
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static inline int
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sr_init(int save_to_disk)
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{
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(void) save_to_disk;
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return 0;
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}
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#endif /* HAVE_MODULE_DIRAUTH */
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```
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The main reason for this approach is to avoid having conditional code
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everywhere in the code base. It should be centralized as much as possible
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which helps maintainability but also avoids conditional spaghetti code
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making the code much more difficult to follow/understand.
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* It is possible that you end up with code that needs to be used by the rest
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of the code base but is still part of your module. As a good example, if
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you look at `src/feature/shared_random_client.c`: it contains code needed
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by the hidden service subsystem but mainly related to the shared random
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subsystem very specific to the dirauth module.
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This is fine but try to keep it as lean as possible and never use the same
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filename as the one in the module. For example, this is a bad idea and
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should never be done:
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- `src/feature/dirclient/shared_random.c`
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- `src/feature/dirauth/shared_random.c`
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* When you include headers from the module, **always** use the full module
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path in your statement. Example:
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`#include "feature/dirauth/dirvote.h"`
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The main reason is that we do **not** add the module include path by default
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so it needs to be specified. But also, it helps our human brain understand
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which part comes from a module or not.
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Even **in** the module itself, use the full include path like above.
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