Arguments and environment variables

Cheerp (nightly) has support for passing arguments and environment variables to your programs. The way you pass them depends on whether you’re compiling for WASI, ES6 or something else.

For all the examples we’ll use the following example program:

#include <stdio.h>

int main(int argc, char **argv, char **env) {
  for (int i = 0; i < argc; ++i)
    printf("arg: %s\n", argv[i]);

  for (int i = 0; env[i]; ++i)
    printf("env: %s\n", env[i]);
}

WASI

Cheerp uses the WASI-CLI interface to get the arguments and environment variables, meaning that it will work on any runtime that has implemented the WASI-CLI proposal.

Here we’ll be using wasmtime:

$ /opt/cheerp/bin/clang -target cheerp-wasm-wasi test.c
$ wasmtime --env A=1 --env B=2 a.out a b c
arg: a.out
arg: a
arg: b
arg: c
env: A=1
env: B=2

ES6 modules

For ES6 modules, you can pass the arguments through the initialization function like this:

import init from "./test.mjs";

init({ argv: ["a", "b", "c"], env: ["A=1", "B=2"] }).then((module) => {});

$ /opt/bin/clang test.c -cheerp-make-module-es6 -o test.mjs
$ node main.mjs
arg: a
arg: b
arg: c
env: A=1
env: B=2

Other

When not building WASI or ES6 modules, you can provide environment variables and program arguments by defining the following global variables in JavaScript before the call to main:

CHEERP_ENV
CHEERP_ARGV

Do note that changes to CHEERP_ENV after main has run will not be reflected in environ.

/opt/bin/clang test.c -cheerp-pretty-code -o test.js

Then we open the file and place the following near the top:

"use strict";
/*Compiled using Cheerp (R) by Leaning Technologies Ltd*/
var __imul = Math.imul;
var __fround = Math.fround;
var CHEERP_ARGV = ["a", "b", "c"];
var CHEERP_ENV = ["A=1", "B=2"];
var oSlot = 0;
var nullArray = [null];
var nullObj = { d: nullArray, o: 0 };
// ...

Then running it:

$ node test.js
arg: a
arg: b
arg: c
env: A=1
env: B=2

NodeJS

When running your program with NodeJS, you can also specify program arguments and/or environment variables like this:

$ node test.js --cheerp-env=A=1 --cheerp-env=B=2 --cheerp-arg=a --cheerp-arg=b --cheerp-arg=c
arg: a
arg: b
arg: c
env: A=1
env: B=2

This will only work if you didn’t pass them in a different way (e.g. using CHEERP_ENV or CHEERP_ARGV)

Limitations

Due to technical reasons, it is currently not possible to receive arguments and/or environment variables in main when the main function is in a different section than the target. For example the following won’t work when compiled with the flag: -target cheerp:

#include <stdio.h>

[[cheerp::wasm]] int main(int argc, char **argv, char **env) {
  for (int i = 0; i < argc; ++i)
    printf("arg: %s\n", argv[i]);

  int i = 0;
  while (env[i])
    printf("env: %s\n", env[i++]);
}

In these cases, you can still get environment variables with the getenv function, the only limitation is that you can’t get them for the arguments to main.