This is WAH, a WebAssembly interpreter implemented in a single C header file. It supports the WebAssembly 3.0 deterministic profile, including the GC, exception handling, relaxed SIMD, multi-memory, memory64/table64, tail calls, and extended const proposals.
WAH is a single-header library. Define WAH_IMPLEMENTATION in exactly one C file to include the implementation:
#define WAH_IMPLEMENTATION
#include "wah.h"
#include <stdio.h>
int main(int argc, char *argv[]) {
// Read a .wasm file into memory
FILE *f = fopen(argv[1], "rb");
fseek(f, 0, SEEK_END);
size_t size = ftell(f);
fseek(f, 0, SEEK_SET);
uint8_t *buf = malloc(size);
fread(buf, 1, size, f);
fclose(f);
// Parse and create an execution context
wah_module_t mod = {0};
wah_exec_context_t ctx = {0};
wah_parse_module(&mod, buf, size, NULL);
wah_exec_context_create(&ctx, &mod, NULL);
free(buf);
// Call an exported function by name
wah_value_t params[2] = {{.i32 = 3}, {.i32 = 4}};
wah_value_t result;
wah_error_t err = wah_call_by_name(&ctx, "add", params, 2, &result);
if (err) {
fprintf(stderr, "error: %s\n", wah_strerror(err));
return 1;
}
printf("add(3, 4) = %d\n", result.i32); // 7
wah_exec_context_destroy(&ctx);
wah_module_destroy(&mod);
return 0;
}WAH passes all currently included files in the official WebAssembly spectest corpus with zero failures (based on spec commit c840c58). One local test-data fix is applied: const.bin.wast corrected per WebAssembly/spec#2150, using regenerated Linux-produced binaries to avoid the upstream generator bug.
Under the deterministic profile, all [!DET]-excluded nondeterminism is eliminated: generated NaN values are canonical and positive, and relaxed SIMD instructions have fixed deterministic behavior. Spec-allowed nondeterminism (memory.grow/table.grow failure due to resource exhaustion) remains.
WAH also comes with a large battery of additional tests covering all major features, edge cases and fault injection scenarios like OOM. Special care was taken to allow untrusted WebAssembly modules to be executed with WAH; it supports both space-based and time-based execution limits and has been fuzzed with multiple harnesses for safety.
WAH is a Public Domain (equivalent) software. The vast majority of its code has been generated by Gemini CLI and Claude Code, albeit with substantial human guidance and fixes, making it a copyrightable work.