共享内存ASharedMemory

• 本文将模拟进程间共享内存的实现，用多线程模拟
  看api描述，他是创建了一个共享的内存区域，并返回一个文件描述符，这个描述符可以被mmap映射
  我们在mmp的时候可以使用flag： PROT_READ | PROT_WRITE | PROT_EXEC
  仅支持android26以上

/**
 * Create a shared memory region.
 *
 * Create shared memory region and returns an file descriptor.  The resulting file descriptor can be
 * mmap'ed to process memory space with PROT_READ | PROT_WRITE | PROT_EXEC. Access to shared memory
 * region can be restricted with {@link ASharedMemory_setProt}.
 *
 * Use close() to release the shared memory region.
 *
 * Use {@link android.os.ParcelFileDescriptor} to pass the file descriptor to
 * another process. File descriptors may also be sent to other processes over a Unix domain
 * socket with sendmsg and SCM_RIGHTS. See sendmsg(3) and cmsg(3) man pages for more information.
 *
 * If you intend to share this file descriptor with a child process after
 * calling exec(3), note that you will need to use fcntl(2) with FD_SETFD
 * to clear the FD_CLOEXEC flag for this to work on all versions of Android.
 *
 * Available since API level 26.
 *
 * \param name an optional name.
 * \param size size of the shared memory region
 * \return file descriptor that denotes the shared memory; -1 and sets errno on failure, or -EINVAL if the error is that size was 0.
 */
int ASharedMemory_create(const char *name, size_t size) __INTRODUCED_IN(26);

1、消费端

• 首先我们构造一个结构体，表示要共享的数据。os_thread_helper用于实现同步机制

  typedef struct utils {
      int id;
      char name[1024];
      struct os_thread_helper oth;
  };
  

• 然后我们为他创建一块共享内存区域，mFd是这块内存的描述符。outPtr是我们消费端对这块内存的mmap映射

  struct utils *mUtil =(struct utils*) malloc(sizeof(struct utils));
  mSharedPtr =  FF::ASharedMemory::create("util",sizeof(mUtil),(void **)&mUtil);
  FileUtilPtr = FF::FileUtil::create();
  FileUtilPtr->setSharedData(mUtil);
  

ASharedMemory::ASharedMemory(const char* name,size_t size,void ** outPtr){
    mFd = ASharedMemory_create(name, size);
    const int access =PROT_READ | PROT_WRITE;
    const int flags =MAP_SHARED;
    void *ptr = mmap(NULL,size,access,flags,mFd,0);
    if(ptr==NULL){
        throw runtime_error("failed to create shared memory");
    }
    *outPtr = ptr;
}

• 我们还需要开启一个线程，循环去这块内存读取数据。消费数据端的工作基本完成了

  void FileUtil::setSharedData(struct utils *util) {
      mUtil = util;
      os_run_func_t func = waitThread;
      os_thread_helper_init(&mUtil->oth);
      os_thread_helper_start(&mUtil->oth, func, mUtil);
  }
  

void* FileUtil::waitThread(void *ptr){
      struct utils *util = reinterpret_cast<struct utils*>(ptr);
    while (os_thread_helper_is_running(&util->oth)){
        if(ptr!=NULL) {
            LOGE("os_thread_helper_wait_locked");
            os_thread_helper_lock(&util->oth);
            os_thread_helper_wait_locked(&util->oth);
            LOGE("readSharedData mUtil.name = %s ,id= %d", util->name, util->id);
            os_thread_helper_unlock(&util->oth);
        }
    }
}

2、生产端

• 接着，我们开始创建生产消息的线程

  这个线程我们在java里面创建，每隔2s往共享内存里面写数据

  class myRunnable implements Runnable {
      @Override
      public void run() {
          while (true) {
              try {
                  Thread.sleep(2000);
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
              clientSendMessage("发送到共享内存时间:" + SystemClock.elapsedRealtime());
          }
      }
  }
  

  接着我们映射了生产端的内存区域，就是mUtilClient。这样我们在mUtilClient里面放置的内容在消费端就能直接拿到，因为是同一块内存区域