Styjun

I am writing a program that takes integers as command line arguments. For each of these numbers I have to create a thread which calculates Fibonacci series up to that number. That function returns a struct pointer to the main where the data is printed.

Now, I have correctly done the fib calculations and checked them by printing the series within the function.

The problem arises when I try to return the struct pointer created within the thread function and use it to print the data in the main.

typedef struct thread_func_param

 int *fib;
 int size;
 thread_func_param;
//=================================================== 
void *fibGen(void *parameters)

 int num = atoi(parameters);
 struct thread_func_param *p;
 p = malloc (sizeof (thread_func_param));

 p->size = fibSize(num);
 p->fib = malloc(sizeof(int)* p->size);

 //Fibonacci Calculations
 //..
 //.

 return (void *) p;
 //pthread_exit((void *) p);

//=================================================== 
int main(int argc, char* argv[])

 void* thread_result;
 thread_func_param* p = malloc( sizeof(thread_func_param));
 assert(argc > 1);

 int noOfThreads = argc - 1;
 printf("No of Thread = %dn", noOfThreads);
 pthread_t *threadID = malloc (sizeof (pthread_t) * noOfThreads);

 pthread_attr_t attributes;
 pthread_attr_init(&attributes);

 int i, j;
 for(i = 0; i < noOfThreads; i++)
 
 pthread_create(&threadID[i], &attributes, fibGen, argv[i+1]);
 pthread_join(threadID[i], thread_result);


 //HOW TO USE THE RETURNED DATA?
 for (j = 0; j< ((thread_func_param*)thread_result->size)-1; j++)
 printf(" %d ", (thread_func_param*)thread_result->fib[j]);
 

 return 0;

The solution that I use, in the end, to print the data gives error of dereferencing a void pointer (I am new with C). How can I correct it?

Two issues here:

1. 
   pthread_join() takes a void** as 2nd parameter. The code passes a void* only.


2. 
   

   To cast a pointer wrap it into parenthesis. Here the cast

   
   
   

   (thread_func_param*)thread_result->size
   

   
   
   

   refers to size not to thread_result. So what you want is

   
   
   

   ((thread_func_param*)thread_result)->size
   

   
   

However a nice and clean solution would only use a void pointer interimswise. It could look like this:

int main(int argc, char* argv[])
{
 thread_func_param* thread_result;

 ...

 ...

 pthread_create(&threadID[i], &attributes, fibGen, argv[i+1]);

 
 void * pv; 
 pthread_join(threadID[i], &pv);
 thread_result = pv;
 

 if (NULL != thread_result) /* perform some sanity checking. */
 
 for (j = 0; j < thread_result->size - 1; j++)
 printf(" %d ", thread_result->fib[j]);
 

 ...