Host API

anvil_plugin_load receives an AnvilHost, a table of functions. It has a size and an api_version field: new functions are appended in minor updates (check size), and a breaking change increases the API major version. The full reference is the SDK header anvil_plugin.h.

Basics

Function	Purpose
log	Write to anvil/anvil.log, tagged with your plugin id.
plugin_dir	Your read-only plugin folder.
data_dir	Your writable folder, anvil/data/<plugin-id>/.
game	Information about the running game: build id (or NULL if unknown), image base and size.

Patching the game

All addresses are RVAs inside eu4.exe. Before writing, the host checks the expected bytes, call target or import, and refuses a patch that overlaps another plugin’s patch; both plugin ids are logged.

Function	Purpose
patch_bytes	Replace bytes at an address.
redirect_call	Point an existing call instruction at your function.
hook_function	Detour a whole function, with a trampoline to call the original.
replace_import	Replace an entry in the game’s import table.
alloc_near_image	Allocate executable memory within ±2 GB of the game image, for your own stubs.

If your plugin’s load fails, all its patches are rolled back.

Services

A plugin can offer a service to other plugins: a C struct of function pointers whose first field is uint32_t size.

host->register_service(self, "my-plugin.math", 1, &g_math);   // provider
const MyMath *math = host->get_service(self, "my-plugin.math", 1);  // consumer

Services are registered under your own id prefix and live for the whole session. Add new functions at the end of the struct (consumers check size); a breaking change gets a new major version, and both majors can be registered at the same time. Publish the struct in a header of your plugin.

Events

subscribe(self, event, callback, context) and publish(self, event, payload) deliver events synchronously on the publishing thread, in subscription order. Built-in events:

Event	When
anvil.plugins_loaded	After the main phase has loaded all plugins.
anvil.game_ready	When the game has finished starting. Only on known builds; check game->ready_signal_known.

The anvil. prefix is reserved for the host.