# Host Functions

_Path: en/wasm/hosts_

## Table of Contents

- Host Functions

## Content

# Host Functions


WASM modules access runtime capabilities through host function imports. Each import is declared explicitly per entry in the `imports` list.



## Import Types


| Import | Description |
|--------|-------------|
| `wasi:cli` | Environment, exit, stdin/stdout/stderr, terminal |
| `wasi:io` | Streams, error handling, polling |
| `wasi:clocks` | Wall clock and monotonic clock |
| `wasi:filesystem` | File system access through mounted directories |
| `wasi:random` | Cryptographically secure random numbers |
| `wasi:sockets` | TCP/UDP networking and DNS resolution |
| `wasi:http` | Outgoing HTTP client requests |

Enable imports in your entry configuration:

```yaml
  - name: my_function
    kind: function.wasm
    fs: myns:assets
    path: /module.wasm
    hash: sha256:...
    method: run
    imports:
      - wasi:cli
      - wasi:io
      - wasi:clocks
      - wasi:filesystem
    pool:
      type: inline
```

Only declare the imports your module actually needs.



## WASI Imports


Each `wasi:*` import enables a group of related WASI Preview 2 interfaces.



### wasi:clocks


**Interfaces:** `wasi:clocks/wall-clock`, `wasi:clocks/monotonic-clock`

Wall clock and monotonic clock for time operations. Monotonic clock integrates with the Wippy dispatcher for async sleep.



### wasi:io


**Interfaces:** `wasi:io/error`, `wasi:io/streams`, `wasi:io/poll`

Stream read/write operations and async polling. The poll interface enables cooperative yielding through the dispatcher.



### wasi:cli


**Interfaces:** `wasi:cli/environment`, `wasi:cli/exit`, `wasi:cli/stdin`, `wasi:cli/stdout`, `wasi:cli/stderr`

Access to environment variables, process exit codes, and standard I/O streams. Environment variables are mapped from the Wippy environment registry through WASI configuration.



### wasi:filesystem


**Interfaces:** `wasi:filesystem/types`, `wasi:filesystem/preopens`

File system access through mounted directories. Mounts are configured per-entry and map Wippy filesystem entries to guest paths.

```yaml
wasi:
  mounts:
    - fs: myns:data
      guest: /data
      read_only: true
```



### wasi:random


**Interfaces:** `wasi:random/random`, `wasi:random/insecure`, `wasi:random/insecure-seed`

Cryptographically secure and insecure random number generation.



### wasi:sockets


**Interfaces:** `wasi:sockets/network`, `wasi:sockets/instance-network`, `wasi:sockets/ip-name-lookup`, `wasi:sockets/tcp`, `wasi:sockets/tcp-create-socket`, `wasi:sockets/udp`

TCP and UDP networking with DNS resolution. Socket operations integrate with the dispatcher for async I/O.



### wasi:http


**Interfaces:** `wasi:http/types`, `wasi:http/outgoing-handler`

Outgoing HTTP client requests from within WASM modules. Supports request/response types defined by the WASI HTTP specification.



## See Also


- [Overview](wasm/overview.md) - WebAssembly runtime overview
- [Functions](wasm/functions.md) - WASM function configuration
- [Processes](wasm/processes.md) - Running WASM as processes



## Navigation

Previous: Functions (wasm/functions)
Next: Processes (wasm/processes)