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Accessing Data in WebAssembly Memory

There are situations where you may need to access WebAssembly memory from your TypeScript code. For example, if you need to allocate a new string, de-reference a pointer, or examine or modify a structure. A TwrLibrary in particular may need to do this. ( For background on the issues involved in using WebAssembly Memory with C and TypeScript see Passing Function Arguments from JavaScript to C/C++ with WebAssembly.).

To access WebAssembly memory you will use the wasmMem public member variable:

  • twrWasmModule has the public member variable wasmMem:IWasmMemory
  • twrWasmModuleAsync has the public member variable wasmMem:IWasmMemoryAsync

If you are writing a twrLibrary, the appropriate wasmMem is the first parameter of your import functions.

Both versions of wasmMem extend IWasmMemoryBase which has common functions for retrieving or setting values from WebAssembly memory. With IWasmMemoryAsync, for functions that call malloc internally, await must be used. This situation arises in the IWasmMemoryAsync versions of the PutXXX functions -- they return a Promise. PutXX makes a call to malloc, and in twrWasmModuleAsync, malloc needs to message the Worker thread and await for a response.

mem8, mem32, memF, and memD can be used to access the WebAssembly memory directly. They are different views on the same memory.

getU8Arr and getU32Arr expect that idx (a pointer) points to a: 

struct {
   int size;
   void* data;
}

Note: In prior versions of twr-wasm, these functions were available directly on the module instance. For example, mod.GetString. These functions have been deprecated. Now you should use mod.wasmMem.getString (for example).

// IWasmMemoryBase operate on shared memory, so they will function in any WasmModule 
export interface IWasmMemoryBase {
   memory:WebAssembly.Memory;
   mem8:Uint8Array;
   mem32:Uint32Array;
   memF:Float32Array;
   memD:Float64Array;
   stringToU8(sin:string, codePage?:number):Uint8Array;
   copyString(buffer:number, buffer_size:number, sin:string, codePage?:number):void;
   getLong(idx:number): number;
   setLong(idx:number, value:number):void;
   getDouble(idx:number): number;
   setDouble(idx:number, value:number):void;
   getShort(idx:number): number;
   getString(strIndex:number, len?:number, codePage?:number): string;
   getU8Arr(idx:number): Uint8Array;
   getU32Arr(idx:number): Uint32Array;
}

// IWasmMemory does not support await, and so will only work in a thread that has the module loaded
// That would be twrWasmModule, twrWasmModuleAsyncProxy
export interface IWasmMemory extends IWasmMemoryBase {
   malloc:(size:number)=>number;
   free:(size:number)=>void;
   putString(sin:string, codePage?:number):number;
   putU8(u8a:Uint8Array):number;
   putArrayBuffer(ab:ArrayBuffer):number;
}

// IWasmMemoryAsync must be used from an async function since await is needed
export interface IWasmMemoryAsync extends IWasmMemoryBase {
   malloc:(size:number)=>Promise<number>;
   free:(size:number)=>Promise<void>;
   putString(sin:string, codePage?:number):Promise<number>;
   putU8(u8a:Uint8Array):Promise<number>;
   putArrayBuffer(ab:ArrayBuffer):Promise<number>;
}