Symbolic breakpoint for when dispatch_async is called with a specific queue

Question

I'm debugging an issue in my project involving grand central dispatch. In debugging this, it would be really helpful to have a way of being notified when work is dispatched to a specific queue.

Is there some way of setting a symbolic breakpoint on dispatch_async with a condition that could check whether the dispatch queue argument is the same as some other queue that I have access to?

Answer 1

Here's how to set a conditional breakpoint. (I haven't done conditions on queues, I'm making the assumption here that pointer equality will Just Work™.)

First get the address of the queue you want, let's say it's 0x12345678. Then create a breakpoint:

breakpoint set -n dispatch_async -c '$register == 0x12345678'

Replace $register with an expression specific to the architecture.

Updated to show $arg1 from Jim Ingham's comment

Simulator

• x86: *(id*)($esp+4)
• x86-64: $arg1 (aka $rdi)

Device

• armv7: $arg1 (aka $r0)
• arm64: $arg1 (aka $x0)

Answer 2

If you set the label on your queue you can just set a conditional breakpoint on a string match within the block being executed. Sometime's I'll just log it like this though.

if (!strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(myDispatchQueue)){
    printf("Booyah!\n");
}