WebAssembly System Interface

The WebAssembly System Interface is a suite of host functions that a Wasm module can import to provide system-level capabilities, such as:

• stdin / stdout / stderr
• environment variables
• command line arguments
• system clock
• random number generation
• basic reading and writing of files (through use of a virtual file system)

All such capabilities are virtualized; i.e., the guest will not have direct access to the corresponding host resources, but they will be mediated by the WASI layer, which can be configured to limit their surface.

How to use

As a host who is running Wasm modules, WASI is just a collection of imports that you need to provide to a wasi-compiled module when instantiating it. You'll also need to configure some options for how these functions behave and what the module can and cannot do.

Bare-Bones Instantiation

In order to instantiate a WASI module you need an instance of WasiPreview1.

For instance, download the following example from the link or with curl:

curl https://raw.githubusercontent.com/dylibso/chicory/main/wasm-corpus/src/main/resources/compiled/hello-wasi.wat.wasm > hello-wasi.wasm

import com.dylibso.chicory.log.SystemLogger;
import com.dylibso.chicory.wasi.WasiOptions;
import com.dylibso.chicory.wasi.WasiPreview1;
import com.dylibso.chicory.wasm.Parser;
import com.dylibso.chicory.runtime.Store;

import java.io.File;

var logger = new SystemLogger();
// let's just use the default options for now
var options = WasiOptions.builder().build();
// create our instance of wasip1
var wasi = new WasiPreview1(logger, WasiOptions.builder().build());
// create the module and connect the host functions
var store = new Store().addFunction(wasi.toHostFunctions());
// instantiate and execute the main entry point
store.instantiate("hello-wasi", Parser.parse(new File("hello-wasi.wasm")));

Note: Notice that we don't explicitly execute the module. The module will run when you instantiate it. This is part of the WASI spec. A WASI module will implicitly call _start. To learn more read this blog post.

stdin, stdout, and stderr

At the very least, you probably want to orchestrate stdin, stdout, and stderr of the module. Often, this is the way you communicate with basic WASI-enabled modules by way of the command pattern. In order to make it easy to manipulate these streams, we expose stdin as an InputStream and stdout/stderr as an OutputStream.

Download from the link or with curl:

curl https://raw.githubusercontent.com/dylibso/chicory/main/wasm-corpus/src/main/resources/compiled/greet-wasi.rs.wasm > greet-wasi.wasm

// Let's create a fake stdin stream with the bytes "Andrea"
var fakeStdin = new ByteArrayInputStream("Andrea".getBytes());
// We will create two output streams to capture stdout and stderr
var fakeStdout = new ByteArrayOutputStream();
var fakeStderr = new ByteArrayOutputStream();
// now pass those to our wasi options builder
var wasiOpts = WasiOptions.builder().withStdout(fakeStdout).withStderr(fakeStderr).withStdin(fakeStdin).build();

var wasi = new WasiPreview1(logger, wasiOpts);

// greet-wasi is a rust program that greets the string passed in stdin
var store = new Store().addFunction(wasi.toHostFunctions());
// instantiating will execute the module if it's a WASI command-pattern module
store.instantiate("hello-wasi", Parser.parse(new File("greet-wasi.wasm")));


// check that we output the greeting
assert(fakeStdout.toString().equals("Hello, Andrea!"));
// there should be no bytes in stderr!
assert(fakeStderr.toString().equals(""));

Notice that it is always possible to connect standard output, standard input and standard error to the system's real streams. For instance, in the case of stdout, you would write:

var wasi = WasiOptions.builder().withStdout(System.out).withStderr(System.err).withStdin(System.in).build();

Supported Features

If your module calls a wasi function that we don't support, or uses a feature that we don't support, we will throw a WasmRuntimeException.

For the most up-to-date info, and to see what specific functions we support, see the WasiPreview1.java and the following table:

WASI Function	Supported	Notes
args_get	✅
args_sizes_get	✅
clock_res_get	🟡	See clock_time_get.
clock_time_get	🟡	Clock IDs process_cputime_id and thread_cputime_id are not supported.
environ_get	✅
environ_sizes_get	✅
fd_advise	✅
fd_allocate	✅
fd_close	✅
fd_datasync	✅
fd_fdstat_get	✅
fd_fdstat_set_flags	✅
fd_fdstat_set_rights	❌
fd_filestat_get	✅
fd_filestat_set_size	✅
fd_filestat_set_times	✅
fd_pread	✅
fd_prestat_dir_name	✅
fd_prestat_get	✅
fd_pwrite	🟡	Not supported for files opened in append mode.
fd_read	✅
fd_readdir	✅
fd_renumber	✅
fd_seek	✅
fd_sync	✅
fd_tell	✅
fd_write	✅
path_create_directory	✅
path_filestat_get	✅
path_filestat_set_times	✅
path_link	❌
path_open	✅
path_readlink	✅
path_remove_directory	✅
path_rename	✅
path_symlink	❌
path_unlink_file	✅
poll_oneoff	❌
proc_exit	✅
proc_raise	💀	This function is no longer part of WASI.
random_get	✅
sched_yield	✅
sock_accept	❌
sock_recv	❌
sock_send	❌
sock_shutdown	✅