Cancelling dispatch_io_read - objective-c

Question

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
}
}

Answer 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.