Using linear memory

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 wasm memory in a module. Be sure to read the documentation for Memory as well.

memory.wat

(module
  (memory (export "memory") 2 3)

  (func (export "size") (result i32) (memory.size))
  (func (export "load") (param i32) (result i32)
    (i32.load8_s (local.get 0))
  )
  (func (export "store") (param i32 i32)
    (i32.store8 (local.get 0) (local.get 1))
  )

  (data (i32.const 0x1000) "\01\02\03\04")
)

memory.c

/*
Example of instantiating of the WebAssembly module and invoking its exported
function.

You can compile and run this example on Linux with:

   cargo build --release -p wasmtime-c-api
   cc examples/memory.c \
       -I crates/c-api/include \
       -I crates/c-api/wasm-c-api/include \
       target/release/libwasmtime.a \
       -lpthread -ldl -lm \
       -o memory
   ./memory

Note that on Windows and macOS the command will be similar, but you'll need
to tweak the `-lpthread` and such annotations.

Also note that this example was taken from
https://github.com/WebAssembly/wasm-c-api/blob/master/example/memory.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);

wasm_memory_t* get_export_memory(const wasm_extern_vec_t* exports, size_t i) {
  if (exports->size <= i || !wasm_extern_as_memory(exports->data[i])) {
    printf("> Error accessing memory export %zu!\n", i);
    exit(1);
  }
  return wasm_extern_as_memory(exports->data[i]);
}

wasm_func_t* get_export_func(const wasm_extern_vec_t* exports, size_t i) {
  if (exports->size <= i || !wasm_extern_as_func(exports->data[i])) {
    printf("> Error accessing function export %zu!\n", i);
    exit(1);
  }
  return wasm_extern_as_func(exports->data[i]);
}


void check(bool success) {
  if (!success) {
    printf("> Error, expected success\n");
    exit(1);
  }
}

void check_call(wasm_func_t* func, const wasm_val_vec_t* args_vec, int32_t expected) {
  wasm_val_t results[1];
  wasm_val_vec_t results_vec = WASM_ARRAY_VEC(results);
  wasm_trap_t *trap = NULL;
  wasmtime_error_t *error = wasmtime_func_call(func, args_vec, &results_vec, &trap);
  if (error != NULL || trap != NULL)
    exit_with_error("failed to call function", error, trap);
  if (results[0].of.i32 != expected) {
    printf("> Error on result\n");
    exit(1);
  }
}

void check_call0(wasm_func_t* func, int32_t expected) {
  wasm_val_vec_t args_vec = WASM_EMPTY_VEC;
  check_call(func, &args_vec, expected);
}

void check_call1(wasm_func_t* func, int32_t arg, int32_t expected) {
  wasm_val_t args[] = { WASM_I32_VAL(arg) };
  wasm_val_vec_t args_vec = WASM_ARRAY_VEC(args);
  check_call(func, &args_vec, expected);
}

void check_call2(wasm_func_t* func, int32_t arg1, int32_t arg2, int32_t expected) {
  wasm_val_t args[] = { WASM_I32_VAL(arg1), WASM_I32_VAL(arg2) };
  wasm_val_vec_t args_vec = WASM_ARRAY_VEC(args);
  check_call(func, &args_vec, expected);
}

void check_ok(wasm_func_t* func, const wasm_val_vec_t* args_vec) {
  wasm_trap_t *trap = NULL;
  wasm_val_vec_t results_vec = WASM_EMPTY_VEC;
  wasmtime_error_t *error = wasmtime_func_call(func, args_vec, &results_vec, &trap);
  if (error != NULL || trap != NULL)
    exit_with_error("failed to call function", error, trap);
}

void check_ok2(wasm_func_t* func, int32_t arg1, int32_t arg2) {
  wasm_val_t args[] = { WASM_I32_VAL(arg1), WASM_I32_VAL(arg2) };
  wasm_val_vec_t args_vec = WASM_ARRAY_VEC(args);
  check_ok(func, &args_vec);
}

void check_trap(wasm_func_t* func, const wasm_val_vec_t* args_vec, size_t num_results) {
  assert(num_results <= 1);
  wasm_val_t results[1];
  wasm_val_vec_t results_vec;
  results_vec.data = results;
  results_vec.size = num_results;
  wasm_trap_t *trap = NULL;
  wasmtime_error_t *error = wasmtime_func_call(func, args_vec, &results_vec, &trap);
  if (error != NULL)
    exit_with_error("failed to call function", error, NULL);
  if (trap == NULL) {
    printf("> Error on result, expected trap\n");
    exit(1);
  }
  wasm_trap_delete(trap);
}

void check_trap1(wasm_func_t* func, int32_t arg) {
  wasm_val_t args[] = { WASM_I32_VAL(arg) };
  wasm_val_vec_t args_vec = WASM_ARRAY_VEC(args);
  check_trap(func, &args_vec, 1);
}

void check_trap2(wasm_func_t* func, int32_t arg1, int32_t arg2) {
  wasm_val_t args[] = { WASM_I32_VAL(arg1), WASM_I32_VAL(arg2) };
  wasm_val_vec_t args_vec = WASM_ARRAY_VEC(args);
  check_trap(func, &args_vec, 0);
}

int main(int argc, const char* argv[]) {
  // Initialize.
  printf("Initializing...\n");
  wasm_engine_t* engine = wasm_engine_new();
  wasm_store_t* store = wasm_store_new(engine);

  // Load our input file to parse it next
  FILE* file = fopen("examples/memory.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, &binary);
  if (error != NULL)
    exit_with_error("failed to parse wat", error, NULL);
  wasm_byte_vec_delete(&wat);

  // Compile.
  printf("Compiling module...\n");
  wasm_module_t* module = NULL;
  error = wasmtime_module_new(engine, &binary, &module);
  if (error)
    exit_with_error("failed to compile module", error, NULL);
  wasm_byte_vec_delete(&binary);

  // Instantiate.
  printf("Instantiating module...\n");
  wasm_instance_t* instance = NULL;
  wasm_trap_t *trap = NULL;
  wasm_extern_vec_t imports = WASM_EMPTY_VEC;
  error = wasmtime_instance_new(store, module, &imports, &instance, &trap);
  if (!instance)
    exit_with_error("failed to instantiate", error, trap);

  // Extract export.
  printf("Extracting exports...\n");
  wasm_extern_vec_t exports;
  wasm_instance_exports(instance, &exports);
  size_t i = 0;
  wasm_memory_t* memory = get_export_memory(&exports, i++);
  wasm_func_t* size_func = get_export_func(&exports, i++);
  wasm_func_t* load_func = get_export_func(&exports, i++);
  wasm_func_t* store_func = get_export_func(&exports, i++);

  wasm_module_delete(module);

  // Try cloning.
  wasm_memory_t* copy = wasm_memory_copy(memory);
  wasm_memory_delete(copy);

  // Check initial memory.
  printf("Checking memory...\n");
  check(wasm_memory_size(memory) == 2);
  check(wasm_memory_data_size(memory) == 0x20000);
  check(wasm_memory_data(memory)[0] == 0);
  check(wasm_memory_data(memory)[0x1000] == 1);
  check(wasm_memory_data(memory)[0x1003] == 4);

  check_call0(size_func, 2);
  check_call1(load_func, 0, 0);
  check_call1(load_func, 0x1000, 1);
  check_call1(load_func, 0x1003, 4);
  check_call1(load_func, 0x1ffff, 0);
  check_trap1(load_func, 0x20000);

  // Mutate memory.
  printf("Mutating memory...\n");
  wasm_memory_data(memory)[0x1003] = 5;
  check_ok2(store_func, 0x1002, 6);
  check_trap2(store_func, 0x20000, 0);

  check(wasm_memory_data(memory)[0x1002] == 6);
  check(wasm_memory_data(memory)[0x1003] == 5);
  check_call1(load_func, 0x1002, 6);
  check_call1(load_func, 0x1003, 5);

  // Grow memory.
  printf("Growing memory...\n");
  check(wasm_memory_grow(memory, 1));
  check(wasm_memory_size(memory) == 3);
  check(wasm_memory_data_size(memory) == 0x30000);

  check_call1(load_func, 0x20000, 0);
  check_ok2(store_func, 0x20000, 0);
  check_trap1(load_func, 0x30000);
  check_trap2(store_func, 0x30000, 0);

  check(! wasm_memory_grow(memory, 1));
  check(wasm_memory_grow(memory, 0));

  wasm_extern_vec_delete(&exports);
  wasm_instance_delete(instance);

  // Create stand-alone memory.
  printf("Creating stand-alone memory...\n");
  wasm_limits_t limits = {5, 5};
  wasm_memorytype_t* memorytype = wasm_memorytype_new(&limits);
  wasm_memory_t* memory2 = wasm_memory_new(store, memorytype);
  check(wasm_memory_size(memory2) == 5);
  check(! wasm_memory_grow(memory2, 1));
  check(wasm_memory_grow(memory2, 0));

  wasm_memorytype_delete(memorytype);
  wasm_memory_delete(memory2);

  // Shut down.
  printf("Shutting down...\n");
  wasm_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);
}