Cast to Pointer Error Multithread Program-CodePudding

This is a multi-threaded program that outputs prime numbers. The user runs the program and enters a number into the command line. It creates a separate thread that outputs all the prime numbers less than or equal to the number entered by the user.

I have an error: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast] I'm so close but I've been staring at this for awhile now. I thought I would get some feedback.

How can I fix this? It is referring to the void here:

(void *)count);

Here is all the code:

#include <stdio.h>
#include <pthread.h>
int N = 100;  //number of promes to be generated
int prime_arr[100000] = {0};  //prime arrray
void *printprime(void *ptr)  //thread function
{
    int j, flag;
    int i = (int)(long long int)ptr;  //getting thread number
    //for thread 0, we check for all primes 0,4,8,12
    //for thread 1, we check for all primes 1,5,9,13
    while (i < N) {  //while number in range
        flag = 0;  //check if i has factor
        for (j = 2; j <= i / 2; j  )  //factor can be at max i/2 value
        {
            if (i % j == 0)  //factor found
            {
                flag = 1;
                break;
            }
        }
        if (flag == 0 && (i > 1))  //prime found, no factor
        {
            prime_arr[i] = 1;
        }
        i  = 4;  //increase by interval of 4
    }
}
int main()
{
    printf("Enter N: ");
    scanf("%d", &N);  //input N
    pthread_t tid[4] = {0};  //create an array of 4 threads
    int count = 0;
    for (count = 0; count < 4; count  )  //initialize threads and start
    {
        printf("\r\n CREATING THREADS %d", count);
        pthread_create(&tid[count], NULL, printprime,(void *)count);  //count is passed as argument, target = printprime
    }
    printf("\n");
    for (count = 0; count < 4; count  )
    {
        pthread_join(tid[count], NULL);  //while all thread havent finished
    }
    int c = 0;
    for (count = 0; count < N; count  )  //print primes
        if (prime_arr[count] == 1)
            printf("%d ", count);
    printf("\n");
   return 0;
}

CodePudding user response：

Use a uintptr_t or a intptr_t instead of an int.

Technically, that's for storing a pointer in an integer, not for storing an integer in a pointer. So it's not exactly kosher. But it's still a common practice.

To do it properly, you would need to (statically or dynamically) allocate a variable for each thread, and pass the address of that variable to the thread.

CodePudding user response：

Here you cast count to a void* which isn't a compatible type.

pthread_create(&tid[count], NULL, printprime, (void*) count);

And here you try to convert it back to an int improperly:

int i = (int)(long long int)ptr;

I suggest creating workpackages, tasks that you instead use and cast proberly to void* and back.

Example:

#include <pthread.h>
#include <stdio.h>

typedef struct {
    pthread_t tid;
    int count;
} task_t;

void *printprime(void *ptr) {
    task_t *task = ptr;
    task->count  = 10;       // do some work
    return NULL;
}

#define TASKS (4)

int main() {
    task_t tasks[TASKS] = {0};              // an array of tasks

    for (int count = 0; count < TASKS;   count) {
        tasks[count].count = count;         // fill the task with some job
        pthread_create(&tasks[count].tid, NULL, printprime, &tasks[count]);
    }

    // join and take care of result from all threads
    for (int count = 0; count < TASKS;   count) {
        pthread_join(tasks[count].tid, NULL);
        printf("task %d value = %d\n", count, tasks[count].count);
    }
}

Demo