Question

I am trying to create a function that returns an object with certain type. The problem is that create thread does not accept it. Can somebody help me with the following code?

struct thread_args
 {
    Key *k;
    QNode *q;
    uint8_t USED_DIMENSION;
};

QLeafNode *st ;
  struct thread_args Structthread2;
     Structthread1.k=min;
     Structthread1.q=start;
      Structthread1.USED_DIMENSION=4 ;

pthread_create( &thread1, NULL,(void*)&FindLeafNode,  ((void *) &Structthread1));
pthread_join( thread1, (void**)st);

QLeafNode* FindLeafNode (Key *k, QNode * r, uint8_t USED_DIMENSION ){

}

Answer 1

First off, your thread function is not correctly defined. Only functions of the form:

void* function_name(void* param)

can be passed to pthread_create.

Now, in order to return a pointer to something from this function, you need two pthread functions:

pthread_exit(void *value_ptr);

Call this inside the thread function to return a value through value_ptr and

pthread_join(pthread_t thread, void **value_ptr);

Call this inside the parent thread to wait for the termination of the child with handle thread and retrieve the value returned by pthread_exit in value_ptr.

So your code should look something like:

struct thread_args
 {
    Key *k;
    QNode *q;
    uint8_t USED_DIMENSION;
};

QLeafNode *st ;
struct thread_args Structthread1;
Structthread1.k=min;
Structthread1.q=start;
Structthread1.USED_DIMENSION=4 ;

pthread_create(&thread1, NULL, FindLeafNode,  ((void *) &Structthread1));
pthread_join(thread1, (void**)st);

...

void* FindLeafNode (void* param) {
    struct thread_args* value = (struct thread_args*) param;
    // use value for computations
    QLeafNode* result = ... // allocate result with new / malloc
    pthread_exit((void*)result);
}