I have a thread created in the main function and PostThreadMessage from ther is invoked with the corresponding thread ID. If one more thread is created in a seperate file how can we invoke PostThreadMessage as we dont know the thread ID which is a parameter for invoking
You have to either:
Store/pass the thread ID (or thread handle) from whatever created the thread to whatever needs to know about the thread; or
Have some way to find the thread via an object it creates. (e.g. If it creates a window with a unique class, you could find that window and then ask the OS which thread owns the window.)
Other than that, there is no magical way to "find a particular thread with no known attributes that was created by another thread that didn't tell anyone about", unless you want to enumerate all threads within your process (but you would have no way to know thread was the right one, unless you did something like #1 or #2 above, and if you do either of them then you don't need to enumerate in the first place).
Note that there will almost always be more threads in your process than the ones you explicitly create, so you cannot just look for "any thread except the two I already know about," because you might pick up a system worker-thread or similar that you should not mess with.
Related
I want to perform some load and save operations on another thread (in SDL). To be able to do this I thought of creating a thread and detaching it (letting it end on its own) everytime I call a function that needs to run separately.
But I don't think this is the correct behaviour (or is it?).
Is there any better solution, like creating and using only one thread? And if there is, how can I call my function(s) from it?
Use std::async. On most implementations it uses efficient solutions like reusing threads from threadpool.
The Life span of a thread is dependent on the main thread(or parent thread), without a join all children threads would be terminated when the main thread(or parent thread) exit.A thread is tied to the process. You might want to looking into forking a process instead, this would persist even if the parent process exit, but would be could be come a zombie process, with no way of terminating it within the program.
what i s the difference between SetEvent() and Thread Lock() function? anyone please help me
Events are used when you want to start/continue processing once a certain task is completed i.e. you want to wait until that event occurs. Other threads can inform the waiting thread about the completion of this task using SetEvent.
On the other hand, critical section is used when you want only one thread to execute a block of code at a time i.e. you want a set of instructions to be executed by one thread without any other thread changing the state at that time. For example, you are inserting an item into a linked list which involves multiple steps, at that time you don't want another thread to come and try to insert one more object into the list. So you block the other thread until first one finishes using critical sections.
Events can be used for inter-process communication, ie synchronising activity amongst different processes. They are typically used for 'signalling' the occurrence of an activity (e.g. file write has finished). More information on events:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms686915%28v=vs.85%29.aspx
Critical sections can only be used within a process for synchronizing threads and use a basic lock/unlock concept. They are typically used to protect a resource from multi-threaded access (e.g. a variable). They are very cheap (in CPU terms) to use. The inter-process variant is called a Mutex in Windows. More info:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms682530%28v=vs.85%29.aspx
I was flicking through the Pickaxe, looking for the documentation on Thread, and came across ThreadGroup.
The documentation describes what it does, but it doesn't explain what it's for.
Is a thread group related to a thread pool, which I assumed Ruby doesn't have?
New threads are created in their parent's ThreadGroup. You can use the ThreadGroup to organize the implicit tree structure given by the parent threads spawning other threads, and use the list instance method to get all threads which have not terminated yet, i.e. to define methods operating on all threads in the group.
Additionaly, you can use enclose to prohibit adding (or removing) threads to this group, if you run untrusted code and want to keep an eye on the threads it spawns.
Noob question:
This link shows an example of CreateEvent and CreateThread
http://msdn.microsoft.com/en-us/library/ms686915(v=vs.85).aspx
My question is if the ThreadProc is truly thread safe?
Specifically, the dwWaitResult variable. Since all threads are waiting on the same event, it turns out this code works but had different events been created, for example, this would not work correct?
The dwWaitResult variable is a local variable in that function. Thus each individual thread has its own copy, which assures that the variable is thread safe. Each thread has its own stack, therefore all local variables are specific to the individual thread.
The Event is created by name, so if the event is already created is reused in any other thread "creating" an event with the same name. As a result, the example code IS thread safe.
I'm making an iPhone app using threads.
I was used C# for a while, there was a method Thread.Join() which blocks current thread for specific thread completes execution. What's the equivalent in Cocoa for it? Or Alternatives?
---edit---
PS. I'm using NSThread.
---edit---
I'm finding a method like 'waitForThreadExit' which blocks caller thread until thread completes execution.
The threads created with Cocoa cannot be created as detached. NSThread instances always wrap attached POSIX threads for resource management reasons. As quoted in the Thread Programming Guide:
If you do want to create joinable
threads, the only way to do so is
using POSIX threads. POSIX creates
threads as joinable by default. To
mark a thread as detached or joinable,
modify the thread attributes using the
pthread_attr_setdetachstate function
prior to creating the thread. After
the thread begins, you can change a
joinable thread to a detached thread
by calling the pthread_detach
function. For more information about
these POSIX thread functions, see the
pthread man page. For information on
how to join with a thread, see the
pthread_join man page.
If you are looking for a way to be notified of the end of a NSThread, you can use the NSThreadWillExitNotification notification.
NSThread does not expose a Join method by any name. NSThread is a very simple, high level, wrapper class. It's very useful for doing threading in a GUI app as it simplifies calling back onto the main thread. For simple backgrounding of tasks and communicating the result back to the main thread on completion this should be sufficient and is fairly easy to get right. If you want to do more "advanced" things (and that includes Join, here) then you'll either have to go to pthreads or layer the semantics on top of NSThread (perhaps by using NSCondition).
If you are using pthreads, then use: pthread_join.
On the other hand, if you are using NSThread class, there is no equivalent to join method you are referring to.
You could try wiht NSObject's message performSelectorOnMainThread:withObject:waitUntilDone:
But I am not exactly sure what you are trying to accomplish here.
Here's is Apple's Multithreading Programming Guide.
You can do this yourself using NSConditionLock. Define two conditions: "running" and "terminated". The worker thread acquires the lock "running" and upon termination it unlocks with condition "terminated". A join would then be to acquire the lock "terminated" and then unlock it "terminated".