pthread, mutex, functions, classes...

theChameleon (12)

hello

how do i make both threads have access to the same set of updated data, when they call different functions?

in addition, is the way i pass in the variable "myObjs" correct? can i pass in STL containers the same way?

example:

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...

class aClass{
...
};

int main(){
    void *plus (void *);
    void *minus (void *);
    pthread_t thread1, thread2;
    ...
    queue<aClass> myObjs;
    ...
    int size = myObjs.size(); //this is OK.
    retVal1 = pthread_create(&thread1, NULL, plus, (void *) myObjs);
    retVal2 = pthread_create(&thread2, NULL, minus, (void *) myObjs);
    ...
}
...

void *plus(void *ptr){
    queue<aClass> *myObjs = (vector<aClass>*) *ptr;
    int size = myObjs.size(); //generates error. how do i resolve this?
}

in this code, thread1 will increase the value of member variables, while thread2 will decrease the value of the member variables... but i need them to be updated.

in addition, when im adding the values, how do i mutex lock the minus function, so it wont add and minus at the same time, and vice versa?

if i change the code to use STL container, i would edit the function "plus()" to add a new object, and the function "minus()" to erase an object.

lastly, how do i use the functions in queue, and the class? it generates an error when it's in the thread function.

the error is:
error: request for member 'size' in 'myObjs', which is non class type 'std::queue<aClass, std:: deque<aClass, std::allocator <aClass>>>*'

Last edited on

theChameleon (12)

the error has been resolved.

so, can somebody help me regarding the things i mentioned in the original post?

Zaita (1607)

how do i make both threads have access to the same set of updated data, when they call different functions?

http://en.wikipedia.org/wiki/Singleton_pattern

in this code, thread1 will increase the value of member variables, while thread2 will decrease the value of the member variables... but i need them to be updated.

See above.

in addition, when im adding the values, how do i mutex lock the minus function, so it wont add and minus at the same time, and vice versa?

When using a class, the mutex becomes a member and can then be used to lock down various sections of code, including methods (minus and plus) that belong to that class.

You can also declare a global mutex if your not going to use OO.

lastly, how do i use the functions in queue, and the class? it generates an error when it's in the thread function.

The way you have tried to do this is a very bad idea. Creating a function local variable and passing it as a reference to the threads.

I will make a few suggestions.

1. Have a look at OO, and how to implement a Singleton-Class.
2. Look at a thread library like Boost that provides easy access to mutexes, barriers and thread creation/management.
3. Don't pass the address of your container through to the thread. Either make it a global variable, or part of a class (ideally a singleton, so you can lock the code when required).

Zaita (1607)

How new are you to C++? Multi-Threaded development is exponentially more difficult that single-threaded development. There are alot of concepts that you have to learn, and you basically have to re-think the design of large parts of your application if you go down the multi-threaded path.

Also, I would have a look at Thread-Specific Storage too. This allows you to have static variables, singleton classes that are 1 per thread, as opposed to 1 per application.

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