Mac应用程序挂起在USB设备的ReadPipe上

我拿了 /Developer/Examples/IOKit/usb/USBSimple Example 并对其进行了修改,使其能够将一些数据传输到(希望很快能从)设备。

void transferData(IOUSBInterfaceInterface245 **intf, UInt8 inPipeRef, UInt8 outPipeRef)
{
    IOReturn err;
    CFRunLoopSourceRef cfSource;

    err = (*intf)->CreateInterfaceAsyncEventSource(intf, &cfSource);
    if(err) {
        printf("transferData: unable to create event source, err = %08x\n", err);
        return;
    }

    // this is what I need to send to the device

    outBuf[0]   = 0;
    outBuf[1]   = 0;
    outBuf[2]   = 0x18;
    [... snip ...]

    // the following works, although I have no confirmation
    // that the data actually arrives to the device but err is 0 afterwards

    err = (*intf)->WritePipe(intf, outPipeRef, outBuf, 64);
    if(err) {
        printf("transferData: WritePipeFailed, err = %08x\n", err);
        return;
    }

    UInt32 numBytesRead;
    numBytesRead = sizeof(inBuf);

    // this hangs until I disconnect the device

    err = (*intf)->ReadPipe(intf, inPipeRef, inBuf, &numBytesRead);
    if(err) {
        printf("transferData: ReadPipeFailed, err = %08x\n", err);
        return;
    }
}

因此,代码的读取部分会一直挂起,直到我在设备输出时断开它 transferData: ReadPipeFailed, err = e00002ed -这证实了我实际上是在说/听设备。

我选择了这种同步方法,而苹果的例子使用异步方法来简化事情,但没有任何效果。设备没有响应吗?但如果没有,该方法是否应该跳过并在numBytesRead中返回0?