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?