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Host Functions

If you find yourself wanting to provide a plug-in with more custom capabilities, beyond what Extism provides by default (loading input, setting output, persisting variables, etc), then Host Functions may be the solution.

In most cases, plug-ins will take some input state from a host, run some logic, and set output for the host to read after the plug-in has finished its execution. But, what if you want your plug-in to be able to interact more deeply with the host environment; maybe you want your plug-in to query the host's database and receive the results? This is where Host Functions become very powerful, creating a more "bi-directional" relationship between host programs and plug-ins. Rather than only the host application invoking plug-in code, Host Functions make it possible for a plug-in to invoke host application code.

A Host Function is essentially a custom function that your host "injects" into a plug-in and appears to the plug-in a WebAssembly "import" or an extern function declaration, for which syntax varies between programming languages. There are examples in each of the supported SDK documentation pages, so please refer to those for specifics about the SDK language you are using.


Currently, the following list of SDKs support Host Functions. Please reach out on Discord or open an issue on GitHub if you'd like to see support added for your language, or if you would like help contributing support!

  • C
  • C++
  • Go
  • Node
  • OCaml
  • PHP
  • Python
  • Rust

Please refer to the SDK documentation for usage about Host Functions by clicking one of the languages listed above.

Function Signatures

A Host Function is comprised of:

  • a name, which identifies the function and is used to call the function from the plug-in
  • input argument types (optional, which must map from the Host SDK language type to a WebAssembly type)
  • output return types (optional, which also map from host language to WebAssembly types)
  • user data object (optional)

Below is a description of the structure of a Host Function in generic pseudo-code:

HostFunction {
// The literal name of the function, how it would be called from a plug-in.
name: string,

// The types of the input arguments/parameters the plug-in caller will provide.
input_param_types: Array<WasmValueType>,

// The types of the output returned from the host function to the plug-in.
output_return_types: Array<WasmValueType>,

// An opaque pointer to an object from the host, accessible to the plug-in.
// NOTE: it is the shared responsibility of the host and plug-in to cast/dereference
// this value properly.
user_data: void*,

The WasmValueType is described in the Data Types section below.

Data Types

Generally, you will need to understand how to translate the data types in your Host Function's signature to the ones available in a PDK from which the function is called. The following table describes these data types.

Host Function TypeRustGoHaskellAssemblyScriptCZigJavaScript
I32i32, isize, u32, usizeint32, uint32, intInt32, Word32i32, isize, u32, usizeint32_t, uint32_ti32, isize, u32, usize
I64i64, u64int64, uint64Int64, Word64i64, u64int64_t, uint64_ti64, u64

= can also be used for any integer size smaller than 32-bit


A value that is passed each time a Host Function is invoked from the plug-in.

It is a handle to data from the host that's available inside the host function. The host and plug-in must agree on the type and functionality of this value. It can be thought of as an opaque pointer.

The UserData value must live at least as long as the Plugin the function is added to.

Calling from Plug-ins

Host Functions must be declared as extern functions within your plug-in code. Doing so depends on the language that the plug-in is developed in. Please see the official documentation for the language for you are using to develop a plug-in.

Example PDK

Here is an example plug-in in Go which calls a Host Function:

package main

import (


// `hello_world` declared below, is a function provided by the host and is linked as a WebAssembly
// "import" available to the plug-in. By adding the `export $function_name` annotation, the TinyGo
// compiler knows that this is an external function.

//export hello_world
func hello_world(x uint64) uint64

//export say_hello
func say_hello() int32 {
input := pdk.InputString()

// Create output, this will be passed through `hello_world`
output := "Hello, " + input
mem := pdk.AllocateString(output)

// Call `hello_world` host function, it accepts an offset in memory and
// returns an offset in memory
offs := hello_world(mem.Offset())
mem = pdk.FindMemory(offs)

return 0

func main() {}

Need help?

If you've encountered a bug or think something is missing, please open an issue on the Extism GitHub repository.

There is an active community on Discord where the project maintainers and users can help you. Come hang out!