Plugins

Plugins in Dovecot are really simple. They basically have two functions:

<plugin_name>_init(module) is called when module_dir_init() is called.
<plugin_name>_deinit() is called when module_dir_deinit() or module_dir_unload() is called.

The <plugin_name> is the short version of the plugin name, based on the filename. For example if the filename is lib11_imap_quota_plugin.so, the <plugin_name> is "imap_quota" and the init function to be called is imap_quota_plugin_init().

Versioning

Since different Dovecot versions can have different APIs, your plugin should usually also define <plugin_name>_version, like:

const char *imap_quota_plugin_version = DOVECOT_ABI_VERSION;

If the version string in plugin doesn't match the version of the running binary, the plugin loading fails. The DOVECOT_ABI_VERSION is defined in Dovecot's config.h, which you're typically including.

Dependencies

Some plugins depend on another one. In some systems (but not all) it's possible to handle this by giving a nicer error message than "symbol xyz not found". There are two steps for this:

First create <plugin_name>_dependencies array listing plugin names that the plugin depends on, like:

const char *imap_quota_plugin_dependencies[] = { "quota", NULL };

Then you'll also have to make the plugin .so binary link to the other plugins:

if PLUGIN_DEPS
lib11_imap_quota_plugin_la_LIBADD = \
        ../quota/lib10_quota_plugin.la
endif

PLUGIN_DEPS is set only if plugin dependencies are actually supported. Otherwise the build might fail or plugin loading might fail.

Once all this is done, trying to load imap_quota plugin without quota plugin gives a nice error message:

Error: Can't load plugin imap_quota_plugin: Plugin quota must be loaded also