How to use a struct pointer returned to the in the main from a thread function? - c

Question

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 = %d\n", 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?

Answer 1

Two issues here:
pthread_join() takes a void** as 2nd parameter. The code passes a void* only.
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]);
}
...