Environment variables

Passing environment variables to your application

Cheerp has support for environment variables.

Providing environment variables

The ways to provide environment variables depend on which target you are compiling to.

JavaScript modules

For JavaScript modules, you can pass environment variables through the initialization function like this:

import init from "./example.js";
await init({
env: ["A=1", "B=2"],


When targeting WASI, Cheerp uses the WASI-CLI interface to recieve environment variables, meaning that it will work on any runtime that has implemented the WASI-CLI proposal.

The exact flag differs between runtimes. With Wasmtime, it’s --env:

Terminal window
$ wasmtime --env A=1 --env B=2 example.wasm


When targeting something else, you can provide environment variables and program arguments by defining a global variable called CHEERP_ENV:

globalThis.CHEERP_ENV = ["A=1", "B=2"];

Changes to CHEERP_ENV after main has run will not be reflected in the environment.

Reading environment variables

In C/C++, environment variables can be read as usual, for example using getenv.

Nonstandard third main parameter

Alternatively, Cheerp supports this nonstandard main signature:

int main(int argc, char **argv, char **env)


It is not possible to receive environment variables in main when the main function is in a different section than the target.

In these cases, you can still get environment variables with the getenv function.

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