Cancelling dispatch_io_read

问题

I'm parsing a very large CSV file using GCD functions (please see code below).

If I encounter an error I'd like to cancel dispatch_io_read. Is there a way to do that?

dispatch_io_read(channel,
            0,
            Int.max,
            dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0))
            { (done, data, error) in

                guard error == 0 else {
                    print("Read Error: \(error)")
                    return
                }

                if done {
                    lineBuffer.dealloc(bufferSize)
                }

                dispatch_data_apply(data)
                    { (region, offset, buffer, size) -> Bool in
                        print(size)

                        let bytes = UnsafePointer<UInt8>(buffer)

                        for var i = 0; i < size; i++ {
                            switch bytes[i] {
                            case self.cr: // ignore \r
                                break
                            case self.lf: // newline
                                lineBuffer[bufferLength] = 0x00 // Null terminated
                                line(line: String(UTF8String: lineBuffer)!)
                                bufferLength = 0
                            case _ where bufferLength < (bufferSize - 1): // Leave space for null termination
                                lineBuffer[bufferLength++] = CChar(bytes[i])
                            default:
                                return false // Overflow! I would like to stop reading the file here.
                            }
                        }

                        return true 
                }
        }

回答1:

Calling dispatch_io_close(DISPATCH_IO_STOP) will cause running dispatch_io_read operations to be interrupted and their handlers to be passed to ECANCELED error (along with partial results), see the dispatch_io_close(3) manpage.

Note that this does not interrupt the actual I/O system calls, it just prevents additional I/O system calls from being entered, so you may have to set an I/O channel high watermark to ensure the appropriate level of I/O granularity for your application.

来源：https://stackoverflow.com/questions/30804903/cancelling-dispatch-io-read