如何在进程之间将文件描述符传递给Ashmem

我正在尝试将指向ashmem区域的文件描述符从Service（进程A）传递到Activity（进程B）。在Service中，我将本机文件描述符放入ParcelFileDescriptor并将其放入捆绑包中，然后通过Messenger发送。但是，当我尝试在Activity中使用此文件描述符进行mmap（）时，得到errno == 9（EBADF，错误的文件号）。

在Service的JNI中创建ashmem区域：

int fd;
int *buff;
JNIEXPORT jint JNICALL Java_com_example_testservice_Test_getTestFD
/* int Test.getTestFD() */
 (JNIEnv * env, jclass jthis) {
    fd = open("/dev/ashmem", O_RDWR); // I couldn't find library with ashmem_create_region
    ioctl(fd, ASHMEM_SET_NAME, "my_mem");
    ioctl(fd, ASHMEM_SET_SIZE, 640*480*12);
    buff = mmap(NULL, 640*480*12, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if(buff == MAP_FAILED)
        return -1;
    buff[0] = 4;
    buff[1] = 5;
    buff[2] = 6;
    return fd;
  }

将fd从服务发送到活动：

int nFD = Test.getTestFD();
ParcelFileDescriptor fd;
try {
    fd = ParcelFileDescriptor.fromFd(nFD);
    Message reply = Message.obtain(this, msg.what, nFD, 0);
    Bundle b = new Bundle(1);
    b.putParcelable("fd", fd);
    reply.setData(b);
    if(msg.replyTo == null) { // Activity asks for FD and we reply to it's message
        Log.e("TestService", "Missing replyTo");
    } else {
        try {
            msg.replyTo.send(reply);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }
} catch (IOException e1) {
    e1.printStackTrace();
}

在活动中接收fd：

Bundle bundle = msg.getData();
ParcelFileDescriptor fd = bundle.getParcelable("fd");
int nFD = fd.getFd();
Log.d("ClientLib", "Received FD (Parcel): "+String.valueOf(nFD));
Log.d("ClientLib", "Received FD (raw): "+String.valueOf(msg.arg1));
int ret = Interface.SetDataFD(nFD);  // implementation below
if(ret != 0)
    Log.e("ClientLib", "SetDataFD (parcel) failed. Error code: "+String.valueOf(ret));

ret = Interface.SetDataFD(msg.arg1);
if(ret != 0)
    Log.e("ClientLib", "SetDataFD (raw) failed. Error code: "+String.valueOf(ret));

在Activity的JNI中处理fd：

int data_fd = -1;
JNIEXPORT
jint JNICALL Java_com_example_clientlib_Interface_SetDataFD
/* int SetDataFD(int fd) */
(JNIEnv * env, jclass jthis, jint fd)
{
    data_fd = fd;
    int * blah = (int*)mmap(NULL, 16, PROT_READ, MAP_SHARED, data_fd, 0);
    if(blah == MAP_FAILED)
        return errno;
    return 0;
}

这是我在logcat中得到的：

D/ClientLib(22104): Received FD (Parcel): 53
D/ClientLib(22104): Received FD (raw): 49
E/ClientLib(22104): SetDataFD (parcel) failed. Error code: 9
E/ClientLib(22104): SetDataFD (raw) failed. Error code: 19

怎么了 我什至不确定传递文件描述符是否有问题，我不知道还能检查什么。