Host Functions¶

Host functions are imports implemented by the embedding application. They are the main capability boundary between guest Wasm and the host process.

Keep host functions small and explicit:

• validate every pointer and length before reading or writing memory,
• cap allocations and output sizes,
• treat handles from the guest as untrusted identifiers,
• expose only the filesystem, network, clock, and random capabilities the guest needs,
• keep blocking host work outside latency-sensitive runtime paths.

Core Wasm imports are low-level. Use them when you control both sides of the boundary or when the ABI is intentionally small. Use WIT and the Component Model when plugin contracts need named records, variants, resources, futures, or streams.

Import Names¶

Wasm imports are addressed by module name and field name. Keep those names stable; changing them is a binary compatibility break for existing guest modules.

If multiple modules need to share the same host functions or guest exports, use a Store and register imports there instead of re-creating import tables for each instance.