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A WASM Plugin in C (wasi-sdk)

The same greeter interface as the Rust, Python, JavaScript, and Go guests, now in C. This is the leanest of all: with no runtime to embed, the component is ~18 KB.

The worked example is the committed fixture tests/wasm-fixtures/greeter-c/, verified by polyglot_c_guest_behaves_identically in crates/fidius-host.

Prerequisites

  • wit-bindgen: cargo install wit-bindgen-cli.
  • wasi-sdk (its bundled clang + wasm32-wasip2 sysroot). Point WASI_SDK at the install (MacPorts: /opt/local/libexec/wasi-sdk).

1. Generate bindings + implement

wit-bindgen c emits a header declaring the functions you must define, named exports_<pkg>_<iface>_<func>. WIT type mapping: strings/lists are { uint8_t *ptr; size_t len; }; result<T, _> is a bool return (true = Ok) with out-params:

wit-bindgen c --world greeter-plugin path/to/greeter/wit --out-dir gen

// greeter_impl.c
#include "greeter_plugin.h"
#include <stdlib.h>
#include <string.h>

void exports_fidius_greeter_greeter_greet(greeter_plugin_string_t *name,
                                          greeter_plugin_string_t *ret) {
    size_t n = 7 + name->len + 1;            // "Hello, " + name + "!"
    uint8_t *buf = malloc(n);
    memcpy(buf, "Hello, ", 7);
    memcpy(buf + 7, name->ptr, name->len);
    buf[n - 1] = '!';
    ret->ptr = buf; ret->len = n;
}

bool exports_fidius_greeter_greeter_add(int64_t a, int64_t b, int64_t *ret,
                                        exports_fidius_greeter_greeter_plugin_error_t *err) {
    *ret = a + b; return true;               // the Ok arm of result<s64, plugin-error>
}

void exports_fidius_greeter_greeter_echo_bytes(greeter_plugin_list_u8_t *data,
                                               greeter_plugin_list_u8_t *ret) {
    uint8_t *buf = malloc(data->len);
    for (size_t i = 0; i < data->len; i++) buf[i] = data->ptr[data->len - 1 - i];
    ret->ptr = buf; ret->len = data->len;
}

bool exports_fidius_greeter_greeter_probe_env(void) { return false; }

uint64_t exports_fidius_greeter_greeter_fidius_interface_hash(void) {
    return 0x0102030405060708ull;            // must equal the other guests' hash
}

Returned buffers are malloc'd; the canonical ABI's cabi_realloc (libc malloc/free) owns them after the call. The host instantiates a fresh sandbox per call, so there is nothing long-lived to leak.

2. Build the component

The wasi-sdk wasm32-wasip2 target links straight to a component (via wasm-component-ld) — no preview1→preview2 adapter needed:

"$WASI_SDK/bin/clang" --target=wasm32-wasip2 -mexec-model=reactor \
  --sysroot="$WASI_SDK/share/wasi-sysroot" \
  -I gen gen/greeter_plugin.c greeter_impl.c gen/greeter_plugin_component_type.o \
  -o greeter_c.wasm

wasm-tools validate --features component-model greeter_c.wasm

Don't pass -O2 if wasm-opt is on PATH

With an optimization flag, the wasi-sdk clang runs wasm-opt (binaryen) as a post-link pass — but binaryen can't parse a Component Model binary and fails with "surprising value (at 0:8)". Omit -O (the guest is tiny), or ensure wasm-opt isn't on PATH for the link.

The greeter_plugin_component_type.o (emitted by wit-bindgen) carries the component's type information; -mexec-model=reactor builds a library (no main).

3. Package, sign, and load

Identical to any fidius package ([wasm].component = "greeter_c.wasm"). The host loads it through the same load_wasm + descriptor as every other guest:

let handle = host.load_wasm("greeter-c-pkg", &Greeter_WASM_DESCRIPTOR)?;
let greeting: String = handle.call_method(0, &("Ada".to_string(),))?;
assert_eq!(greeting, "Hello, Ada!");

See also