How do I access the memory as a byte array?

[jakob999999]

Hello,

I was wondering if anyone is aware of how to access the memory as a byte array from the wasm_exec_env_t in C?

I have seen someone do it in wasmer/wasi and javascript like this:

let bytes = new Uint8Array(instance.exports.memory.buffer);

Thanks for any help in advance.

[lum1n0us]

If coding internal has to access WASMModuleInstance first, then WASMMemoryInstance is the target.

If coding in host functions, need to

• export the memory
• access the memory data with a Wasm C API named wasm_memory_data()

[jakob999999]

Thank you. I am stuck on how to get the wasm_memory_t* pointer that is used internally from the wasm_module_inst_t.

[lum1n0us]

wasm_memory_xxx is partial of wasm-c-api. Should follow the declarations in the header [https://github.com/bytecodealliance/wasm-micro-runtime/blob/main/core/iwasm/include/wasm_c_api.h]. Better not hack wasm_module_inst_t.

Btw, there is an example.

[jakob999999]

Thanks! I was wondering if it is possible to initialize a wasm runtime with the memory from a wasm_instance_t? If I initialize a wasm runtime I am using the wasm_module_inst_t but then I do not have the pointer to the wasm_memory_t like I have with the wasm_instance_t.

[wenyongh]

@jakob999999 A possible method to access the memory from exec_env:

wasm_module_inst_t module_inst;
uint32 app_start_offset, app_end_offset;
void *mem_start_addr, *mem_end_addr;

module_inst = wasm_runtime_get_module_inst(exec_env);

/* Use offset 0 to get the begin/end offset of wasm app */
ret = wasm_runtime_get_app_addr_range(module_inst, 0, &app_start_offset, &app_end_offset);
if (!ret) { return error; }

/* Convert app_start_offset/app_end_offset to native address */
mem_start_addr = wasm_runtime_addr_app_to_native(module_inst, app_start_offset);
mem_end_addr = wasm_runtime_addr_app_to_native(module_inst, app_end_offset);

And note that native address of linear memory may be changed after opcode memory.grow is executed, had better get the start/end address each time before accessing the linear memory.