using dispatch_sync in Grand Central Dispatch

Question

Can anyone explain with really clear use cases what the purpose of dispatch_sync in GCD is for? I can\'t understand where and why I would have to u

Answer 1

Here's a half-way realistic example. You have 2000 zip files that you want to analyze in parallel. But the zip library isn't thread-safe. Therefore, all work that touches the zip library goes into the unzipQueue queue. (The example is in Ruby, but all calls map directly to the C library. "apply", for example, maps to dispatch_apply(3))

#!/usr/bin/env macruby -w

require 'rubygems'
require 'zip/zipfilesystem'

@unzipQueue = Dispatch::Queue.new('ch.unibe.niko.unzipQueue')
def extractFile(n)
    @unzipQueue.sync do
        Zip::ZipFile.open("Quelltext.zip") {   |zipfile|
            sourceCode = zipfile.file.read("graph.php")
        }
    end
end

Dispatch::Queue.concurrent.apply(2000) do |i|
   puts i if i % 200 == 0
   extractFile(i)
end