Setting a timeout on ifstream in C++?

问题:

We're trying to read data from 2 usb mice connected to a linux box (this data is used for odometry/localization on a robot). So we need to continuously read from each mouse how much it moved. The problem is that when a mouse is not moving, it doesn't send any data, so the file stream from which we get the data blocks execution and therefore the program can't do the odometry calculations (which involve time measurement for speed).

Is there a way to set a timeout on the input stream (we're using ifstream in C++ and read from /dev/input/mouse), so that we're able to know when the mouse doesn't move, instead of waiting for an event to be received? Or do we need to mess up with threads (arggh...)? Any other suggestions are welcome!

Thanks in advance!

回答1:

A common way to read from multiple file descriptors in linux is to use select(). I suggest starting with the manpage. The basic system flow is as follows:

1) Initialize devices
2) Obtain list of device file descriptors
3) Setup the time out
4) Call select with file descriptors and timeout as parameters - it will block until there is data on one of the file descriptors or the time out is reached
5) Determine why select returned and act accordingly (i.e. call read() on the file descriptor that has data). You may need to internally buffer the result of read until an entire data gram is obtained.
6) loop back to 4.

This can become your programs main loop. If you already have a different main loop you, can run the above without looping, but your will need to insure that the function is called frequently enough such that you do not lose data on the serial ports. You should also insure that your update rate (i.e. 1/timeout) is fast enough for your primary task.

Select can operate on any file descriptor such network sockets and anything else that exposes an interface through a file descriptor.

回答2:

What you're looking for would be an asynchronous way to read from ifstream, like socket communication. The only thing that could help would be the readsome function, perhaps it returns if no data is available, but I doubt this helps.

Using threads would be the best way to handle this.

回答3:

Take a look at the boost Asio library. This might help you deal with the threading suggested by schnaeder.

回答4:

No, there is no such method. You'll have to wait for an event, or create a custom Timer class and wait for a timeout to repoll, or use threads.