C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  URGENT !! QUEUE STL PROBLEM URGENT!!          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Sachin Jagtap Guest Posted: Thu Apr 28, 2005 7:20 am    Post subject: URGENT !! QUEUE STL PROBLEM URGENT!! Hi, I am getting exception while poping item from queue, I am writing messages coming from client in a queue in one thread and then in other thread i am reading from queue and writing in file. I have not implemented any syncronization between reading and wrinting, just i checking size of queue, if it is not empty i am reading from queue and wrting to file. somethign like class CMessage { public: string strmessage; }; thread 1 { CMessage objMessage; queue.push(objMessage); } thread2 { if(size of queue is not empty) { CMessage objmessage = queue.front(); //SOMETIMES I AM GETTING EXCEPTION HERE. queue.pop(); WriteInFile(); } } Back to top Mark Stijnman Guest Posted: Thu Apr 28, 2005 8:33 am    Post subject: Re: URGENT !! QUEUE STL PROBLEM URGENT!! Whenever one object is manipulated by more than one thread, you should provide locking around the object. The problem is that focus can shift from one thread to another at any time. This means there is no guarantee whatsoever, for instance, that the thread will never switch somewhere halfway the queue.push command. In multi-threading lingo: queue.push is not guaranteed to be atomic. Suppose that command is implemented to first increase the length of the queue, and only after that assign the new element. If the control passes to the other thread between those two operations, it may find the queue non-empty, but without an element assigned. I can't be sure that's what really happening in your case (I don't know the specifics of the implementation that you're using), but it's quite likely to be something similar. Use locks or mutexes to make sure only one thread accesses an object at the same time (reading at the same time is fine, but when one thread writes to an object, all other threads *must* wait until it's done). Good luck, grtz Mark Sachin Jagtap wrote: Quote: Hi, I am getting exception while poping item from queue, I am writing messages coming from client in a queue in one thread and then in other thread i am reading from queue and writing in file. I have not implemented any syncronization between reading and wrinting, just i checking size of queue, if it is not empty i am reading from queue and wrting to file. somethign like class CMessage { public: string strmessage; }; thread 1 { CMessage objMessage; queue.push(objMessage); } thread2 { if(size of queue is not empty) { CMessage objmessage = queue.front(); //SOMETIMES I AM GETTING EXCEPTION HERE. queue.pop(); WriteInFile(); } } Back to top Sachin Jagtap Guest Posted: Fri Apr 29, 2005 8:04 am    Post subject: Re: URGENT !! QUEUE STL PROBLEM URGENT!! Hi Mark, Thanks for your reply I am having only one reader and one writer thread, do I still need to use mutex? If not what could be the possible reason for crash. But with one reader and one writer how can I synchronize this. As reader and writer are two diff threads and there is no common function between them. Thanks!! regards, Sachin "Mark Stijnman" <m.a.stijnman (AT) tnw (DOT) utwente.nl> wrote Quote: Whenever one object is manipulated by more than one thread, you should provide locking around the object. The problem is that focus can shift from one thread to another at any time. This means there is no guarantee whatsoever, for instance, that the thread will never switch somewhere halfway the queue.push command. In multi-threading lingo: queue.push is not guaranteed to be atomic. Suppose that command is implemented to first increase the length of the queue, and only after that assign the new element. If the control passes to the other thread between those two operations, it may find the queue non-empty, but without an element assigned. I can't be sure that's what really happening in your case (I don't know the specifics of the implementation that you're using), but it's quite likely to be something similar. Use locks or mutexes to make sure only one thread accesses an object at the same time (reading at the same time is fine, but when one thread writes to an object, all other threads *must* wait until it's done). Good luck, grtz Mark Sachin Jagtap wrote: Hi, I am getting exception while poping item from queue, I am writing messages coming from client in a queue in one thread and then in other thread i am reading from queue and writing in file. I have not implemented any syncronization between reading and wrinting, just i checking size of queue, if it is not empty i am reading from queue and wrting to file. somethign like class CMessage { public: string strmessage; }; thread 1 { CMessage objMessage; queue.push(objMessage); } thread2 { if(size of queue is not empty) { CMessage objmessage = queue.front(); //SOMETIMES I AM GETTING EXCEPTION HERE. queue.pop(); WriteInFile(); } } Back to top Alvin Guest Posted: Fri Apr 29, 2005 11:58 am    Post subject: Re: URGENT !! QUEUE STL PROBLEM URGENT!! Sachin Jagtap wrote: Quote: Hi Mark, Thanks for your reply I am having only one reader and one writer thread, do I still need to use mutex? If not what could be the possible reason for crash. But with one reader and one writer how can I synchronize this. As reader and writer are two diff threads and there is no common function between them. Thanks!! regards, Sachin Use a mutex to synchronise them. The writer locks the mutes, do the writing, then unlocks the mutex. If the reader locks the mutex, reads, then unlocks the mutex. The synchronisation occurs when one attempts to lock the mutex while the other had it already locked. For exmaple, if the reader tries to lock the mutex after the writer had already locked it, then the reader will wait for the mutex to be unlock (i.e. when the writer unlocks the mutex). This examples is just a simplification, I suggest reading up on Multi-Threaded Programming to get the full picture. -- Alvin Back to top Mark Stijnman Guest Posted: Fri Apr 29, 2005 12:08 pm    Post subject: Re: URGENT !! QUEUE STL PROBLEM URGENT!! Sachin Jagtap wrote: Quote: Hi Mark, Thanks for your reply I am having only one reader and one writer thread, do I still need to use mutex? If not what could be the possible reason for crash. But with one reader and one writer how can I synchronize this. As reader and writer are two diff threads and there is no common function between them. Thanks!! regards, Sachin It doesn't matter that the threads don't share any functions, they share an -object- and its -data-. They both operate on the same set of data, in this case a queue. Adding an element to the queue will change the data. Actually, now that I think of it, the other thread also changes the data when popping one element from the queue. So both threads can change the data embedded in the queue object, and there is no guarantee that these operations will not be interrupted by switching threads - remember, the OS can switch the active thread whenever it wants. So if one thread is halfway pushing, then the active thread switches to the other, and then that thread pops an element, the queue object can easily be left in an undefined state. Or the other way around: halfway popping an object, the other thread might try to push a new one into the queue, leaving again undefined state of the queue. So yes, you have to add mutexes wherever you access a shared object. It *should* help - whenever you have a program that works single-threaded and suddenly stops working when you add an extra thread, it's almost always due to shared data being accessed in a wrong way. 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