Matt Matt - 1 month ago 12
Swift Question

Swift dispatch_async results in EXC_BAD_ACCESS error

In my Swift project, I'm trying to process a FIFO queue (I will call it a list here to avoid confusion) in a background thread. When I use dispatch_async, it results in a EXC_BAD_ACCESS error after only some of the list has been executed.
I've simplified the code as much as I could into the following playground code.
In the playground, when main_thread is set to true, the code processes all 100 items in the list. If it's false, only a handful of items get processed. If the code is in a project, EXC_BAD_ACCESS occurs when main_thread is false. Obviously, I've also tried specifying a serial queue but that doesn't seem to help.
What am I missing or not understanding?

import UIKit

let main_thread = false
let serial_queue = true

class main_class {
var this_list = list_class()

func run(){
for i in 1...100 {
this_list.add_to_list( String(i) )

if main_thread {
} else {
if serial_queue {
let my_serial_queue = dispatch_queue_create("msq", DISPATCH_QUEUE_SERIAL)
dispatch_async(my_serial_queue){()->Void in
} else {
dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), { () ->Void in


class list_class {
var my_list: [String] = []

func add_to_list( this: String ){
my_list.append( this )

func process_list(){
if my_list.count > 0 {
print("removing " + my_list[0])
remove_from_list( my_list[0] )

func remove_from_list( this: String ){
let found = my_list.indexOf( this )
if found != nil {
my_list.removeAtIndex( found! )

var blah = main_class()


Your main thread is exiting while the background thread is still running. Main thread ends -> process gets terminated.

Set up something for main to block on while the background thread is still operating. For example, use dispatch_group_wait(); along with dispatch_group_enter() and dispatch_group_leave().