Using multi-value

You can also browse this source code online and clone the wasmtime repository to run the example locally.

This example shows off how to interact with a wasm module that uses multi-value exports and imports.

multi.wat

(module (func $f (import "" "f") (param i32 i64) (result i64 i32)) (func $g (export "g") (param i32 i64) (result i64 i32) (call $f (local.get 0) (local.get 1)) ) (func $round_trip_many (export "round_trip_many") (param i64 i64 i64 i64 i64 i64 i64 i64 i64 i64) (result i64 i64 i64 i64 i64 i64 i64 i64 i64 i64) local.get 0 local.get 1 local.get 2 local.get 3 local.get 4 local.get 5 local.get 6 local.get 7 local.get 8 local.get 9) ) 

multi.c

/* Example of instantiating of the WebAssembly module and invoking its exported function. You can build using cmake: mkdir build && cd build && cmake .. && cmake --build . --target wasmtime-multi Also note that this example was taken from https://github.com/WebAssembly/wasm-c-api/blob/master/example/multi.c originally */ #include <inttypes.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <wasm.h> #include <wasmtime.h> static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap); // A function to be called from Wasm code. wasm_trap_t *callback(void *env, wasmtime_caller_t *caller, const wasmtime_val_t *args, size_t nargs, wasmtime_val_t *results, size_t nresults) { printf("Calling back...\n"); printf("> %" PRIu32 " %" PRIu64 "\n", args[0].of.i32, args[1].of.i64); printf("\n"); results[0] = args[1]; results[1] = args[0]; return NULL; } // A function closure. wasm_trap_t *closure_callback(void *env, wasmtime_caller_t *caller, const wasmtime_val_t *args, size_t nargs, wasmtime_val_t *results, size_t nresults) { int i = *(int *)env; printf("Calling back closure...\n"); printf("> %d\n", i); results[0].kind = WASMTIME_I32; results[0].of.i32 = (int32_t)i; return NULL; } int main(int argc, const char *argv[]) { // Initialize. printf("Initializing...\n"); wasm_engine_t *engine = wasm_engine_new(); wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL); wasmtime_context_t *context = wasmtime_store_context(store); // Load our input file to parse it next FILE *file = fopen("examples/multi.wat", "r"); if (!file) { printf("> Error loading file!\n"); return 1; } fseek(file, 0L, SEEK_END); size_t file_size = ftell(file); fseek(file, 0L, SEEK_SET); wasm_byte_vec_t wat; wasm_byte_vec_new_uninitialized(&wat, file_size); if (fread(wat.data, file_size, 1, file) != 1) { printf("> Error loading module!\n"); return 1; } fclose(file); // Parse the wat into the binary wasm format wasm_byte_vec_t binary; wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, &binary); if (error != NULL) exit_with_error("failed to parse wat", error, NULL); wasm_byte_vec_delete(&wat); // Compile. printf("Compiling module...\n"); wasmtime_module_t *module = NULL; error = wasmtime_module_new(engine, (uint8_t *)binary.data, binary.size, &module); if (error) exit_with_error("failed to compile module", error, NULL); wasm_byte_vec_delete(&binary); // Create external print functions. printf("Creating callback...\n"); wasm_functype_t *callback_type = wasm_functype_new_2_2(wasm_valtype_new_i32(), wasm_valtype_new_i64(), wasm_valtype_new_i64(), wasm_valtype_new_i32()); wasmtime_func_t callback_func; wasmtime_func_new(context, callback_type, callback, NULL, NULL, &callback_func); wasm_functype_delete(callback_type); // Instantiate. printf("Instantiating module...\n"); wasmtime_extern_t imports[1]; imports[0].kind = WASMTIME_EXTERN_FUNC; imports[0].of.func = callback_func; wasmtime_instance_t instance; wasm_trap_t *trap = NULL; error = wasmtime_instance_new(context, module, imports, 1, &instance, &trap); if (error != NULL || trap != NULL) exit_with_error("failed to instantiate", error, trap); wasmtime_module_delete(module); // Extract export. printf("Extracting export...\n"); wasmtime_extern_t run; bool ok = wasmtime_instance_export_get(context, &instance, "g", 1, &run); assert(ok); assert(run.kind == WASMTIME_EXTERN_FUNC); // Call. printf("Calling export...\n"); wasmtime_val_t args[2]; args[0].kind = WASMTIME_I32; args[0].of.i32 = 1; args[1].kind = WASMTIME_I64; args[1].of.i64 = 2; wasmtime_val_t results[2]; error = wasmtime_func_call(context, &run.of.func, args, 2, results, 2, &trap); if (error != NULL || trap != NULL) exit_with_error("failed to call run", error, trap); // Print result. printf("Printing result...\n"); printf("> %" PRIu64 " %" PRIu32 "\n", results[0].of.i64, results[1].of.i32); assert(results[0].kind == WASMTIME_I64); assert(results[0].of.i64 == 2); assert(results[1].kind == WASMTIME_I32); assert(results[1].of.i32 == 1); // Shut down. printf("Shutting down...\n"); wasmtime_store_delete(store); wasm_engine_delete(engine); // All done. printf("Done.\n"); return 0; } static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) { fprintf(stderr, "error: %s\n", message); wasm_byte_vec_t error_message; if (error != NULL) { wasmtime_error_message(error, &error_message); wasmtime_error_delete(error); } else { wasm_trap_message(trap, &error_message); wasm_trap_delete(trap); } fprintf(stderr, "%.*s\n", (int)error_message.size, error_message.data); wasm_byte_vec_delete(&error_message); exit(1); }