What is the difference between GCD Dispatch Sources and select()?

Question

I've been writing some code that replaces some existing:

while(runEventLoop){
  if(select(openSockets, readFDS, writeFDS, errFDS, timeout) > 0){
    // check file descriptors for activity and dispatch events based on same 
  }
} 

socket reading code. I'd like to change this to use a GCD queue, so that I can pop events on to the queue using dispatch_async instead of maintaining a "must be called on next iteration" array. I also am already using a GCD queue to /contain/ this particular action, hence wanting to devolve it to a more natural GCD dispatch form. ( not a while() loop monopolizing a serial queue )

However, when I tried to refactor this into a form that relied on dispatch sources fired from event handlers tied to DISPATCH_SOURCE_TYPE_READ and DISPATCH_SOURCE_TYPE_WRITE on the socket descriptors, the library code that depended on this scheduling stopped working. My first assumption is that I'm misunderstanding the use of DISPATCH_SOURCE_TYPE_READ and DISPATCH_SOURCE_TYPE_WRITE - I had assumed that they would yield roughly the same behavior as calling select() with those socket descriptors.

Do I misunderstand GCD dispatch sources? Or, regarding the refactor, am I using it in a situation where it is not best suited?

Answer 1

The short answer to your question is: none. There are no differences, both GCD dispatch sources and select() do the same thing: they notify the user that a specific kernel event happened or that a particular condition holds true.

Note that, on a mac or iOS device you should not use select(), but rather the more advanced kqueue() and kevent() (or kevent64()).

You may certainly convert the code to use GCD dispatch sources, but you need to be careful not to break other code relying on this. So, this needs a complete inspection of the whole code handling signals, file descriptors, socket and all of the other low level kernel events.

May be a simpler solution could be to maintain the original code, simply adding GCD code in the part that react to events. Here, you dispatch events on different queues depending on the particular type of event.