Fox’s Thread Scheduler


This documentation and API is ALPHA and is subject to change. Alpha APIs are not under semantic versioning and can change freely between versions.

This covers more technical details of Fox’s thread scheduler. While not essential to know, reading this can provide insight to potential quirks of the implementation.


Fox’s scheduling system has two goals: be easily injectable to existing code, and to control thread execution order. It implements a pthread-like interface by relying mostly on the system pthreads implementation. There are specific functions that have custom implementations:

  • thread creation and destruction to track threads.
  • locks and mutexes to be cooperatively scheduled.
  • fox-specific APIs to control thread execution (yielding, scheduling, etc.)

Then, mach_override is used to replace pthread methods. Currently, the following functions can be assumed to be overridden (although, it may not always):

  • pthread functions (threads, mutexes, cond variables)
  • POSIX semaphores (that are not deprecated by OS X or iOS)
  • Mach semaphores
  • OSSpinLock

These overrides are global for the entire process. That means libraries will also trigger Fox’s custom code when hooks take place. If you find apis that should be supported file an issue.

Currently, Fox’s function replacement is permanent. A flag is set internally to trigger the original code or Fox’s custom code. While this detail is minor, it has a possibility of creating differences in execution time and preemptive scheduling when not using Fox’s scheduler.


Using Fox does not override usage of explicit kernel threads.

Using the Scheduler

The public API of Fox’s scheduler is purposefully kept small. FOXScheduler is the public interface to Fox’s C scheduler. Let’s look at the methods:

@interface FOXScheduler : NSObject

- (instancetype)initWithRandom:(id<FOXRandom>)random;
- (instancetype)initWithRandom:(id<FOXRandom>)random
- (void)runAndWait:(void(^)())block;


-[initWithRandom:] is recommended to use the majority of the time. It simply calls through to -[initWithRandom:random replaceSystemFunctions:YES]. Random is used for indirectly dictating the ordering of threads for the scheduler to run.

When replaceSystemFunctions is YES, then Fox will use mach_override to replace the system functions. Using NO will require manually cooperation for threads and locks.

Use -[runAndWait:] to activate Fox’s scheduler for the given block. The method will block until all the threads finishes executing. Note that runAndWait: only captures threads inside the block. This can be problematic for GCD queues, which can create threads.

In order for cooperative scheduling to work properly, FOXSchedulerYield() needs to be calls throughout all the threads to “mark” break points.

Due to implementation reasons of NSThread, the scheduler cannot force a thread to yield immediately. Instead, manually insert an explicit yield to pause a thread at startup:

NSThread *thread = [[NSThread alloc] initWithTarget:myObject
[thread start];

// implementation of run
- (void)run {
  // do work.

Not doing this will cause threads to run in parallel immediately. FOXRunParallelProgram() does this automatically.

Cooperative Scheduling

Fox’s threading implementation is a user-level, cooperatively scheduled library. User-level means it’s not implemented in terms of the OS. Cooperatively scheduled means threads must explicitly yield execution to another thread (unlike normal threads which get preemptively yielded by the scheduler).

Preemptive scheduling is complex for programs - usually require complex signal handling for some basic reliability and does not allow Fox to retain full execution control.

Fox’s threads aren’t particularly useful for anything other than testing. In fact, they’re actually slower that normal thread or serial execution.

Fox’s threading library “serializes” all thread execution (like event-IO or fiber libraries). Fox can control the order of execution of threads with a custom thread scheduler. This gives more control to help make parallel tests more deterministic.

Of course, Fox’s scheduler works hand-in-hand with the Foxling Compiler to avoid having to manually insert thread yields (which is very error-prone).