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

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?

alk · Accepted Answer · 2019-03-24T16:04:56.560

2

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]);
    }

    ...

edited Mar 24 '19 at 16:04

answered Mar 24 '19 at 15:31

alk

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Corrected the code. Now, it gives a segmentation fault. – Robin Goodfellow Mar 24 '19 at 15:38
In which line does the code end with a segmentation violation? – alk Mar 24 '19 at 15:41
In gdb, it is on the line where I declare the for loop. It cannot match the condition for i. – Robin Goodfellow Mar 24 '19 at 15:48
As an aside, Why did you recommend using `pthread_exit` and not `return`. – Robin Goodfellow Mar 24 '19 at 16:00
At SO we say thank you by upvoting an answer. And if its the right answer by accepting it by clicking its check-mark. – alk Mar 24 '19 at 16:01
"*Why did you recommend ...*" I removed it. Doing `return p;` does the same. – alk Mar 24 '19 at 16:05
1

Of course I was going to do it; I was making some changes in my code. – Robin Goodfellow Mar 24 '19 at 16:12

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

1 Answers1