WebAssembly 中的多线程

Question

如果您回答我关于 WebAssembly 多线程的问题，我将不胜感激。
我想用 2 个线程(主线程和辅助线程)实现代码，这样就有一个全局变量用作辅助线程中的计数器变量，并在循环中递增它。和主线程，读取计数器变量，一次在运行指令之前，一次在运行指令之后(测量完成该指令所需的时间)。
我已经实现了这个代码:

#include "pthread.h"
#include <stdio.h>
#include <unistd.h>
#include<chrono>

int i;
int counter;

void* timerfunction( void *ptr)
{
  printf ("Thread Timer!\n");
  //cout<<"Thread Timer!"<<endl;
  while(1)
  {
    counter=counter+1;
  }
  pthread_exit("The thread was exited!");
}




int main()
{
    pthread_t thread_id;
    void *thread_result;
    int c=0;
    int l=pthread_create(&thread_id,NULL,timerfunction,&c);
    int t1= counter;//reading the counter for the first one

    //intended instruction that we want to measure its execution time   

    int t2= counter;//reading the counter for the second one
    int t3 = t2 - t1;//computing the time
    printf ("value in the counter is: %d \n", t3);
    return 0;
}

我的理解是多线程对Wasm的支持是不完整的，因为它没有同时运行主线程和其他线程，它需要像sleep之类的东西来进行线程之间的切换。因此，我们不能将多线程 Wasm 用于某些目标，例如在一个线程中增加计数器并在另一个线程中同时读取它。我的问题是我的推论是真的还是假的？如果是真的，有什么问题？从 C 或编译过程还是...？是否有使用完整多线程的替代方法？
非常感谢。

Answer 1

你很幸运，Emscripten has implemented PThreads with shared memory .

有一些警告

As of Sep 2019, some browsers have disabled SharedArrayBuffer due to the Spectre set of vulnerabilities. Until it is restored you can still experiment with it if you flip a pref in those browsers. In other browsers (like Chrome on desktop), SharedArrayBuffer is fully enabled by default and you don’t need to flip any flags.

Its a mechanism to create timers used in Specter/Meltdown attacks

Note that SharedArrayBuffer was disabled by default in all major browsers on 5 January, 2018 in response to Spectre. Chrome re-enabled it in v67 on platforms where its site-isolation feature is enabled to protect against Spectre-style vulnerabilities.

我没有测试过，但以下可能有效

# Assuming you have a makefile, the following might work
sudo docker run --rm -v $(pwd):/src trzeci/emscripten emmake make
sudo docker run --rm -v $(pwd):/src trzeci/emscripten emcc \
src/main.o \
-s ALLOW_MEMORY_GROWTH=1 \
-s ERROR_ON_UNDEFINED_SYMBOLS=0 \
-s USE_PTHREADS=1 \