Processes & Concurrency

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Discover how the operating system manages multiple applications and processes simultaneously, ensuring smooth multitasking performance.

Concurrency Documentation

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BGContinuedProcessingTask compatibility with background URLSession
My app does really large uploads. Like several GB. We use the AWS SDK to upload to S3. It seemed like using BGContinuedProcessingTask to complete a set of uploads for a particular item may improve UX as well as performance and reliability. When I tried to get BGContinuedProcessingTask working with the AWS SDK I found that the task would fail after maybe 30 seconds. It looked like this was because the app stopped receiving updates from the AWS upload and the task wants consistent updates. The AWS SDK always uses a background URLSession and this is not configurable. I understand the background URLSession runs in a separate process from the app and maybe that is why progress updates did not continue when the app was in the background. Is it expected that BGContinuedProcessingTask and background URLSession are not really compatible? It would not be shocking since they are 2 separate background APIs. Would the Apple recommendation be to use a normal URLSession for this, in which case AWS would need to change their SDK? Or does Apple think that BGContinuedProcessingTask should just not be used with uploads? In other words use an upload specific API. Thanks!
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Aug ’25
SSMenuAgent consuming lots of CPU
My load average on a largely idle system is around 22, going up to 70 or so periodically; SSMenuAgent seems to be consuming lots of CPU (and, looking at spindump, it certainly seems busy), but... it's not happening on any other system whose screens I am observing. (Er, I know about load average limitations, the process is also consuming 70-98% CPU according to both top and Activity Monitor.) Since this machine (although idle) has our network extension, I'm trying to figure out if this is due to that, or of this is generally expected. Anyone?
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422
May ’25
Service Management Resources
Service Management framework supports installing and uninstalling services, including Service Management login items, launchd agents, and launchd daemons. General: Forums subtopic: App & System Services > Processes & Concurrency Forums tag: Service Management Service Management framework documentation Daemons and Services Programming Guide archived documentation Technote 2083 Daemons and Agents — It hasn’t been updated in… well… decades, but it’s still remarkably relevant. EvenBetterAuthorizationSample sample code — This has been obviated by SMAppService. SMJobBless sample code — This has been obviated by SMAppService. Sandboxing with NSXPCConnection sample code WWDC 2022 Session 10096 What’s new in privacy introduces the new SMAppService facility, starting at 07˸07 BSD Privilege Escalation on macOS forums post Getting Started with SMAppService forums post Background items showing up with the wrong name forums post Related forums tags include: XPC, Apple’s preferred inter-process communication (IPC) mechanism Inter-process communication, for other IPC mechanisms Share and Enjoy — Quinn “The Eskimo!” @ Developer Technical Support @ Apple let myEmail = "eskimo" + "1" + "@" + "apple.com"
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2.2k
Sep ’25
Background Refresh Stalls After Charging on watchOS 26
Hello everyone, I’m a new developer still learning as I go. I’m building a simple watchOS app that tracks Apple Watch battery consumption, records hourly usage data, and uses that information to predict battery life in hours. I’ve run into an issue where background refresh completely stalls after charging and never recovers, regardless of what I do. The only way to restore normal behavior is to restart the watch. Background refresh can work fine for days, but if the watch is charging and a scheduled background refresh tries to run during that period, it appears to be deferred—and then remains in that deferred state indefinitely. Even reopening the app or scheduling new refreshes doesn’t recover it. Has anyone else encountered this behavior? Is there a reliable workaround? I’ve seen a few reports suggesting that there may be a regression in scheduleBackgroundRefresh() on watchOS 26, where tasks are never delivered after certain states. Any insights or confirmations would be greatly appreciated. Thank you!
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176
Oct ’25
BGContinuedProcessingTask UI
When I use BGContinuedProcessingTask to submit a task, my iPhone 12 immediately shows a notification banner displaying the task’s progress. However, on my iPhone 15 Pro Max, there’s no response — the progress UI only appears in the Dynamic Island after I background the app. Why is there a difference in behavior between these two devices? Is it possible to control the UI so that the progress indicator only appears when the app moves to the background?
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173
Oct ’25
What is ImmersiveSpaceAppModel in BOT-anist?
I would like to implement an expression that pops out from the window to Immersive based on the following WWDC video. This video introduces the new features of visionOS 2.0 in the form of refurbishing Apple's sample app BOT-anist. https://developer.apple.com/jp/videos/play/wwdc2024/10153/?time=1252 In the video, it looks like ImmersiveSpaceAppModel is newly implemented. However, the key code is not mentioned anywhere. You pass appModel.robot as the from argument to the transform method of RealityViewContent. It seems that BOT-anist has been updated once and can be downloaded from the following URL, but there is no class such as ImmersiveSpaceAppModel implemented in this app either. https://developer.apple.com/documentation/visionos/bot-anist Has it been further updated again? Frankly, I'm not sure if it is possible to proceed as per the WWDC video. Translated with DeepL.com (free version)
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454
Jan ’25
System Network Extension XPC with LaunchAgent
I've discovered that a system network extension can communicate with a LaunchDaemon (loaded using SMAppService) over XPC, provided that the XPC service name begins with the team ID. If I move the launchd daemon plist to Contents/Library/LaunchAgents and swap the SMAppService.daemon calls to SMAppService.agent calls, and remove the .privileged option to NSXPCConnection, the system extension receives "Couldn't communicate with a helper application" as an error when trying to reach the LaunchAgent advertised service. Is this limitation by design? I imagine it is, but wanted to check before I spent any more time on it.
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213
Jul ’25
Service Showing "Not Responding" in Activity Monitor Despite Running Threads.
I am encountering an issue with my application, BloxOneEndpoint.pkg, which includes two services: rc_service_infoblox – Runs as the root user. Controller Application – Runs as a normal user. Although a thread within rc_service_infoblox is running fine and performing its expected tasks, I notice that the service appears as "Not Responding" in Activity Monitor. Despite normal functionality, this status is concerning, as it may indicate some issue to customer. I would appreciate any insights into why this might be happening and how to resolve it. Is there a specific API or mechanism I should use to ensure the service remains in a "Running" state in Activity Monitor? Thank you for your guidance.
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210
Apr ’25
Using raise in GCD can cause timing issues with the signal mechanism.
when we use raise in GCD, the signal handler is executed asynchronously, whereas in pthread, it is executed synchronously as expected. example: #include <Foundation/Foundation.h> #include <pthread/pthread.h> static void HandleSignal(int sigNum, siginfo_t* signalInfo, void* userContext) { printf("handle signal %d\n", sigNum); printf("begin sleep\n"); sleep(3); printf("end sleep\n"); } void InstallSignal(void) { static const int g_fatalSignals[] = { SIGABRT, SIGBUS, SIGFPE, SIGILL, SIGPIPE, SIGSEGV, SIGSYS, SIGTRAP, }; int fatalSignalsCount = sizeof(g_fatalSignals) / sizeof(int); struct sigaction action = {{0}}; action.sa_flags = SA_SIGINFO | SA_ONSTACK; #if defined(__LP64__) action.sa_flags |= SA_64REGSET; #endif sigemptyset(&action.sa_mask); action.sa_sigaction = &HandleSignal; struct sigaction pre_sa; for(int i = 0; i < fatalSignalsCount; i++) { int sigResult = sigaction(g_fatalSignals[i], &action, &pre_sa); } } void* RaiseAbort(void *userdata) { raise(SIGABRT); printf("signal handler has finished\n"); return NULL; } int main(int argc, const char * argv[]) { InstallSignal(); dispatch_async(dispatch_get_global_queue(0, 0), ^{ raise(SIGABRT); // abort(); // abort() is ok RaiseAbort(nullptr); }); // pthread is ok // pthread_t tid; // int ret = pthread_create(&tid, NULL, RaiseAbort, NULL); // if (ret != 0) { // fprintf(stderr, "create thread failed\n"); // return EXIT_FAILURE; // } [[NSRunLoop mainRunLoop] run]; return 0; } console log: signal handler has finished handle signal 6 begin sleep end sleep
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Sep ’25
dispatch_main and main thread local variables
Hello, We're seeing some strange crashes and noticed the following. It's unclear if related or not. The contract for xpc_main, which internally calls dispatch_main, is This function never returns. and they are appropriately peppered with __attribute__((__noreturn__)). Documentation states: This function “parks” the main thread and waits for blocks to be submitted to the main queue. However, internally, dispatch_main calls pthread_exit. pthread_exit's documentation states that: After a thread has terminated, the result of access to local (auto) variables of the thread is undefined. Thus, references to local variables of the exiting thread should not be used for the pthread_exit() value_ptr parameter value. I'd say the two contracts of This function never returns. and thread exiting with its storage released are diametrically opposed and can create nuanced issues. Consider the following code: struct asd { int a; }; struct asd* ptr; void fff(void* ctx) { while(true) { printf("%d\n", ptr->a); ptr->a = (ptr->a + 1); usleep(100000); } } int main(int argc, const char * argv[]) { struct asd zxc; zxc.a = 1; ptr = &zxc; dispatch_async_f(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), NULL, fff); dispatch_main(); return 0; } This is a gross over-simplification of the code we have, but in the same "spirit". We have a C++ object that is created on the stack and exposes one of its members as a global pointer, with the assumption that it would never release. What I understand from This function never returns is that the calling thread remains dormant and its stack remains alive. What I understand from pthread_exit is that the thread is killed (this is verified with a debugger attached) and its stack storage is released. Another thing that is throwing me off is that no sanitizer that is provided by clang/Xcode catches this issue. I don't see any special handling of the internal pthread_t in libdispatch to keep the stack storage alive. Our code is more complex, but can be solved by allocating the initial object on the heap, rather than on the stack. But still I would like to understand if this is the expected behavior. Perhaps my preconception of __attribute__((__noreturn__)) is wrong, and accessing stack variables post call to a __attribute__((__noreturn__)) function is UB? Thanks
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Jul ’25
Expected behavior from apps when MacOS goes to sleep
Every time macOS goes to sleep the processes get suspended which is expected. But during the sleep period, all processes keep coming back and they all get a small execution window where they make some n/w requests. Regardless of what power settings i have. It also does not matter whether my app is a daemon or not Is there any way that i can disable this so that when system is in sleep, it stays in suspended, no intermittent execution window? I have tried disabling Wake for network access setting but processes still keep getting intermittent execution window. Is there any way that i can prevent my app from coming back while in sleep. I don't want my app to get execution window, perform some executions and then get suspended not knowing when it will get execution window again?
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111
Aug ’25
BGContinuedProcessingTask code pauses when device is locked
I have been experimenting with the BGContinuedProcessingTask API recently (and published sample code for it https://github.com/infinitepower18/BGContinuedProcessingTaskDemo) I have noticed that if I lock the phone, the code that runs as part of the task stops executing. My sample code simply updates the progress each second until it gets to 100, so it should be completed in 1 minute 40 seconds. However, after locking the phone and checking the lock screen a few seconds later the progress indicator was in the same position as before I locked it. If I leave the phone locked for several minutes and check the lock screen the live activity says "Task failed". I haven't seen anything in the documentation regarding execution of tasks while the phone is locked. So I'm a bit confused if I encountered an iOS bug here?
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1w
App is getting terminated by OS even after calling BG task expiration
The application is placed into the idle state. Subsequently, the device enters a sleep state. While the device is in sleep, App start background task within the application successfully receives its expirationHandler callback. App received the expiration callback and App called the end BGtask OS did not released the Assertion. Resulting in App getting terminated by the OS for exceeding the BG task Apple Feedback- FB19192371
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105
Aug ’25
Handling XPC Communication to Multiple Clients: Is Storing Connections a Reliable Approach?
This is the functionality I am trying to achieve with libxpc: There's one xpc server and two xpc clients. When the xpc server receives a particular dictionary item from clientB, the server needs to send a response to both clientA and clientB. This is the approach I am currently using: First, clientA creates a dictionary item that indicates that this item is from clientA. Now, clientA sends this dictionary to server. When server receives this item, it stores the connection instance with clientA in a global variable. Next, when clientB sends a particular dictionary item, server uses this global variable where it perviously stored clientA's connection instance to send a response back to clientA, alongside clientB. Only one edge case I can see is that when clientA closes this connection instance, server will be trying to send a response to an invalidated connection. Question: Is this approach recommended? Any edge cases I should be aware of? Is there any better way to achieve this functionality?
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446
Feb ’25
Is background processing even possible?
Hello, aspiring programmer here. I am developing a StepCounter APP, which keeps track of how many steps I have taken and sends to an MQTT server. I am trying to make this happen even while the app is not in focus, but so far I have not been able to get this working. First tried with silent background music, which seemed pretty inconsistent and inpractical, since I usually play youtube videoes while walking, making the app stop with its silent audio. Then tried GPS, which didnt really do anything (could be implementation problem). Has anyone made background processing work for their apps?
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109
Mar ’25
How to correctly deploy bundled launchdaemons/launchagents?
I'm working on an enterprise product that's mainly a daemon (with Endpoint Security) without any GUI component. I'm looking into the update process for daemons/agents that was introduced with Ventura (Link), but I have to say that the entire process is just deeply unfun. Really can't stress this enough how unfun. Anyway... The product bundle now contains a dedicated Swift executable that calls SMAppService.register for both the daemon and agent. It registers the app in the system preferences login items menu, but I also get an error. Error registering daemon: Error Domain=SMAppServiceErrorDomain Code=1 "Operation not permitted" UserInfo={NSLocalizedFailureReason=Operation not permitted} What could be the reason? I wouldn't need to activate the items, I just need them to be added to the list, so that I can control them via launchctl. Which leads me to my next question, how can I control bundled daemons/agents via launchctl? I tried to use launchctl enable and bootstrap, just like I do with daemons under /Library/LaunchDaemons, but all I get is sudo launchctl enable system/com.identifier.daemon sudo launchctl bootstrap /Path/to/daemon/launchdplist/inside/bundle/Library/LaunchDaemons/com.blub.plist Bootstrap failed: 5: Input/output error (not super helpful error message) I'm really frustrated by the complexity of this process and all of its pitfalls.
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829
Aug ’25
How to correctly access and handle background operations on IOS
Hello, aspiring programmer here. I am developing a StepCounter APP, which keeps track of how many steps I have taken and sends to an MQTT server. I am trying to make this happen even while the app is not in focus, but so far I have not been able to get this working. First tried with silent background music, which seemed pretty inconsistent and inpractical, since I usually play youtube videoes while walking, making the app stop with its silent audio. Then tried GPS, which didnt really do anything (could be implementation problem). Has anyone made background processing work for their apps?
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139
Mar ’25
BGTaskScheduler fails to match unique identifiers to a registered wildcard handler for BGContinuedProcessingTask
Testing Environment: iOS Version: 26.0 Beta 7 Xcode Version: 17.0 Beta 6 Device: iPhone 16 Pro Description: We are implementing the new BGContinuedProcessingTask API and are using the wildcard identifier notation as described in the official documentation. Our Info.plist is correctly configured with a permitted identifier pattern, such as com.our-bundle.export.*. We then register a single launch handler for this exact wildcard pattern. We are performing this registration within a UIViewController, which is a supported pattern as BGContinuedProcessingTask is explicitly exempt from the "register before applicationDidFinishLaunching" requirement, according to the BGTaskScheduler.h header file. The register method correctly returns true, indicating the registration was successful. However, when we then try to submit a task with a unique identifier that matches this pattern (e.g., com.our-bundle.export.UUID), the BGTaskScheduler.shared.submit() call throws an NSInternalInconsistencyException and terminates the app. The error reason is: 'No launch handler registered for task with identifier com.our-bundle.export.UUID'. This indicates that the system is not correctly matching the specific, unique identifier from the submit call to the registered wildcard pattern handler. This behavior contradicts the official documentation. Steps to Reproduce: Create a new Xcode project. In Signing & Capabilities, add "Background Modes" (with "Background processing" checked) and "Background GPU Access". Add a permitted identifier (e.g., "com.company.test.*") to BGTaskSchedulerPermittedIdentifiers in Info.plist. In a UIViewController's viewDidLoad, register a handler for the wildcard pattern. Check that the register method returns true. Immediately after, try to submit a BGContinuedProcessingTaskRequest with a unique identifier that matches the pattern. Expected Results: The submit call should succeed without crashing, and the task should be scheduled. Actual Results: The app crashes immediately upon calling submit(). The console shows an uncaught NSInternalInconsistencyException with the reason: 'No launch handler registered for task with identifier com.company.test.UUID'. Workaround: The issue can be bypassed if we register a new handler for each unique identifier immediately before submitting a request with that same unique identifier. This strongly suggests the bug is in the system's wildcard pattern-matching logic.
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128
Sep ’25
Helper app is sandboxed (entitlement + runtime check), but `URLsForDirectory:` returns user home (`/Users//`) instead of container path — why?
Problem summary I have a macOS helper app that is launched from a sandboxed main app. The helper: has com.apple.security.app-sandbox = true and com.apple.security.inherit = true in its entitlements, is signed and embedded inside the main app bundle (placed next to the main executable in Contents/MacOS), reports entitlement_check = 1 (code signature contains sandbox entitlement, implemented via SecStaticCode… check), sandbox_check(getpid(), NULL, 0) returns 1 (runtime sandbox enforcement present), APP_SANDBOX_CONTAINER_ID environment variable is not present (0). Despite that, Cocoa APIs return non-container home paths: NSHomeDirectory() returns /Users/<me>/ (the real home). [[NSFileManager defaultManager] URLsForDirectory:inDomains:] and URLForDirectory:inDomain:appropriateForURL:create:error: return paths rooted at /Users/<me>/ (not under ~/Library/Containers/<app_id>/Data/...) — i.e. they look like non-sandboxed locations. However, one important exception: URLForDirectory:... for NSItemReplacementDirectory (temporary/replacement items) does return a path under the helper's container (example: ~/Library/Containers/<app_id>/Data/tmp/TemporaryItems/NSIRD_<helper_name>_hfc1bZ). This proves the sandbox is active for some FileManager APIs, yet standard directory lookups (Application Support, Documents, Caches, and NSHomeDirectory()) are not being redirected to the container. What I expect The helper (which inherits the sandbox and is clearly sandboxed) should get container-scoped paths from Cocoa’s FileManager APIs (Application Support, Documents, Caches), i.e. paths under the helper’s container: /Users/<me>/Library/Containers/<app_id>/Data/.... What I tried / diagnostics already gathered Entitlements & code signature codesign -d --entitlements :- /path/to/Helper.app/Contents/MacOS/Helper # shows com.apple.security.app-sandbox = true and com.apple.security.inherit = true Runtime checks (Objective-C++ inside helper): extern "C" int sandbox_check(pid_t pid, const char *op, int flags); NSLog(@"entitlement_check = %d", entitlement_check()); // SecStaticCode check NSLog(@"env_variable_check = %d", (getenv("APP_SANDBOX_CONTAINER_ID") != NULL)); NSLog(@"runtime_sandbox_check = %d", sandbox_check(getpid(), nullptr, 0)); NSLog(@"NSHomeDirectory = %s", NSHomeDirectory()); NSArray *urls = [[NSFileManager defaultManager] URLsForDirectory:NSApplicationSupportDirectory inDomains:NSUserDomainMask]; NSLog(@"URLsForDirectory: %@", urls); Observed output: entitlement_check = 1 env_variable_check = 0 runtime_sandbox_check = 1 NSHomeDirectory = /Users/<me> URLsForDirectory: ( "file:///Users/<me>/Library/Application%20Support/..." ) Temporary/replacement directory (evidence sandbox active for some APIs): NSURL *tmpReplacement = [[NSFileManager defaultManager] URLForDirectory:NSItemReplacementDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:YES error:&err]; NSLog(@"NSItemReplacementDirectory: %@", tmpReplacement.path); Observed output (example): /Users/<me>/Library/Containers/<app_id>/Data/tmp/TemporaryItems/NSIRD_<helper_name>_hfc1bZ Other facts Calls to NSHomeDirectory() and URLsForDirectory: are made after main() to avoid "too early" initialization problems. Helper is placed in Contents/MacOS (not Contents/Library/LoginItems). Helper is a non-GUI helper binary launched by the main app (not an XPC service). macOS version: Sequoia 15.6 Questions Why do NSHomeDirectory() and URLsForDirectory: return the real /Users/<me>/... paths in a helper process that is clearly sandboxed (entitlement + runtime enforcement), while NSItemReplacementDirectory returns a container-scoped temporary path? Is this behavior related to how the helper is packaged or launched (e.g., placement in Contents/MacOS vs Contents/Library/LoginItems, or whether it is launched with posix_spawn/fork+exec vs other APIs)? Are there additional entitlements or packaging rules required for a helper that inherits sandbox to have Cocoa directory APIs redirected to the container (for Application Support, Documents, Caches)? *Thanks in advance — I can add any requested logs
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160
Sep ’25