Emscripten 中的"模块"变量在编译 Pthread 给 worker 时的机制

Question

在将Pthread编译为Web Worker+Wasm 时，我被Emsripten中的Module变量弄糊涂了

有一个简单的Pthread代码，它只是在每个线程的共享变量sum中添加1。(最后附上simple.c。(

我们可以使用命令编译Pthread代码：

$ emcc simple.c  -s USE_PTHREADS=1  -s PTHREAD_POOL_SIZE=4 -o simple.html

Emscripten将生成simple.html、simple.js、simple.worker.js和simple.wasm。

在simple.worker.js中，有一个片段：

// simple.worker.js
var Module = {};
// ...
Module['instantiateWasm'] = function(info, receiveInstance) {
// Instantiate from the module posted from the main thread.
// We can just use sync instantiation in the worker.
var instance = new WebAssembly.Instance(Module['wasmModule'], info);
// We don't need the module anymore; new threads will be spawned from the main thread.
Module['wasmModule'] = null;
receiveInstance(instance); // The second 'module' parameter is intentionally null here, we don't need to keep a ref to the Module object from here.
return instance.exports;
};

请注意，它在worker中声明var Module = {}，并定义Module['instantiateWasm']。

然而，Module['instantiateWasm']仅由simple.js调用，代码片段如下所示：

//simple.js
var Module = {}
// ...
if (Module['instantiateWasm']) {
try {
var exports = Module['instantiateWasm'](info, receiveInstance);
return exports;
} catch(e) {
err('Module.instantiateWasm callback failed with error: ' + e);
return false;
}
}
// ...

正如我们所看到的，simple.js也声明了var Module = {}。

AFAIK、VAR全局变量不能跨主线程及其工作线程访问我不明白为什么simple.js可以调用Module['instantiateWasm']，因为simple.js的Module和simple.worker.js的Module应该不是一回事

---

p线程代码：

// simple.c
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUMTHRDS 4
#define MAGNIFICATION 1e9
typedef struct
{
int thread_id;
double *sum;
} Arg;
pthread_t callThd[NUMTHRDS];
pthread_mutex_t mutexsum;
void *count_pi(void *arg)
{
Arg *data = (Arg *)arg;
int thread_id = data->thread_id;
double *sum = data->sum;
pthread_mutex_lock(&mutexsum);
*sum += 1;
pthread_mutex_unlock(&mutexsum);
printf("Thread %d: sum=%fn", thread_id, *sum);
pthread_exit((void *)0);
}
int main(int argc, char *argv[])
{
pthread_mutex_init(&mutexsum, NULL);
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
double *sum = malloc(sizeof(*sum));
*sum = 0;
Arg arg[NUMTHRDS];
for (int i = 0; i < NUMTHRDS; i++)
{
arg[i].thread_id = i;
arg[i].sum = sum;
pthread_create(&callThd[i], &attr, count_pi, (void *)&arg[i]);
}
pthread_attr_destroy(&attr);
void *status;
for (int i = 0; i < NUMTHRDS; i++)
{
pthread_join(callThd[i], &status);
}
printf("Final Sum =  %f n", *sum);
free(sum);
pthread_mutex_destroy(&mutexsum);
pthread_exit(NULL);
}

Answer 1

主程序将self发送给worker。

// simple.js
// Ask the new worker to load up the Emscripten-compiled page. This is a heavy operation.
worker.postMessage({
'cmd': 'load',
// If the application main .js file was loaded from a Blob, then it is not possible
// to access the URL of the current script that could be passed to a Web Worker so that
// it could load up the same file. In that case, developer must either deliver the Blob
// object in Module['mainScriptUrlOrBlob'], or a URL to it, so that pthread Workers can
// independently load up the same main application file.
'urlOrBlob': Module['mainScriptUrlOrBlob'] || _scriptDir,
'wasmMemory': wasmMemory,
'wasmModule': wasmModule,
'DYNAMIC_BASE': DYNAMIC_BASE,
'DYNAMICTOP_PTR': DYNAMICTOP_PTR
});

而工人进口。

// simple.worker.js
if (typeof e.data.urlOrBlob === 'string') {
importScripts(e.data.urlOrBlob);
} else {
var objectUrl = URL.createObjectURL(e.data.urlOrBlob);
importScripts(objectUrl);
URL.revokeObjectURL(objectUrl);
}

因此Module不是共享的，而是独立初始化的。