# ffi

Description: ffi cheatcode documentation

Note: This document was authored using MDX

  Source: https://github.com/NomicFoundation/hardhat-website/tree/main/src/content/docs/docs/reference/cheatcodes/External/ffi.mdx

{/* This document contains content copied/adapted from the Foundry Book (MIT licensed). See LICENSE in the parent directory. */}

### Signature

```solidity
function ffi(string[] calldata) external returns (bytes memory);
```

### Description

Calls an arbitrary command if [`ffi`](https://hardhat.org/docs/reference/configuration#solidity-tests-configuration) is enabled.

It is generally advised to use this cheatcode as a last resort, and to not enable it by default, as anyone who can change the tests of a project will be able to execute arbitrary commands on devices that run the tests.

### Tips

- By default the `ffi` cheatcode assumes the output of the command is a hex encoded value (e.g. a hex string of an ABI encoded value). If hex decoding fails, it will return the output as UTF8 bytes that you can cast to a string.
- Make sure that the output does not include a `\n` newline character. (e.g in Rust use `print!` vs `println!`)
- Remember that the script will be executed from the top-level directory of your project, not inside `test`
- Make sure that the inputs array does not have empty elements. They will be handled as inputs by the script, instead of space
- Use the cheatcode `toString` to easily convert arbitrary data to strings, so that you can pass them as command-line arguments
- On Windows some commands like `npm` or `npx` fail to execute with `program not found` error message. To avoid such make sure the command contains file extension as well, like `npm.cmd` or `npx.cmd`. These commands can be configured and read as environment variables, so tests doesn't need to be changed when running on different operating systems.

### Examples

ABI encoded output

```solidity
string[] memory inputs = new string[](3);
inputs[0] = "echo";
inputs[1] = "-n";
// ABI encoded "gm", as a hex string
inputs[2] = "0x00000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000002676d000000000000000000000000000000000000000000000000000000000000";

bytes memory res = vm.ffi(inputs);
string memory output = abi.decode(res, (string));
assertEq(output, "gm");
```

UTF8 string output

```solidity
string[] memory inputs = new string[](3);
inputs[0] = "echo";
inputs[1] = "-n";
inputs[2] = "gm";

bytes memory res = vm.ffi(inputs);
assertEq(string(res), "gm");
```