Create an Int Array from Int Array Address

Question

I am currently working my way through threads in C, and I am currently stuck on using my one parameter (an array to be sorted) on thread creation as the array to work with. For an example:

void* sort(void* param)

How do I grab param so that it can be used as an int array local variable?

Also to return the sorted array would I just do return array on the last line,

and do something like int sorted_array[] = pthread_create(&tid, &attr, sort, &un_sorted_array) to capture it?

I'm new to C to any help would be appreciated? Using void pointer to an array The solution here still gave me an invalid initalizer error.

Possible duplicate of Concept of void pointer in C programming — Tim Biegeleisen
– Tim Biegeleisen, Commented Oct 12, 2015 at 0:21
void* merge(void* arg) {int *arg_ptr[] = (int*) arg; int arr[] = *arg_ptr;} gives me an invaild initializer error — atg
– atg, Commented Oct 12, 2015 at 0:39
Try void* merge(void* arg) { int *arg_ptr = (int*) arg; }. To get the result you need to call pthread_join. — Nikolay K
– Nikolay K, Commented Oct 12, 2015 at 4:22

Nikolay K · Accepted Answer · 2015-10-12 04:42:34Z

To access your array you can cast void pointer to int pointer like this

void* merge(void* arg) {
    int* a = (int*) arg;
}

To return array you simply do

return a;

where a is int*. And to get the result from main thread you call pthread_join with second argument that points to some int*.

pthread_join(tid, (void*)&b);

where b is int* b;.

To pass argument to thread you need to get address of the first element of the array (it is the same as pointer for this array). Do it like this

pthread_create(&tid, NULL, merge, (void*)&a[0]);

where a is an array int a[N];.

See full example for pthread with int array as argument and int* as return value.

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

#define N 10

void* merge(void* arg) {
    int* arr = (int*) arg;
    int* res = malloc(N * sizeof(int));
    for(int i = 0; i < N; ++i) {
            res[i] = arr[i] + 1;
    }
    return res;
}

int main() {
    pthread_t tid;

    int a[N];
    int* b;
    for(int i = 0; i < N; ++i) {
            a[i] = i;
    }

    printf("a = [");
    for(int i = 0; i < N; ++i) {
            printf("%3d ", a[i]);
    }
    printf("]\n");

    pthread_create(&tid, NULL, merge, (void*)&a[0]);
    pthread_join(tid, (void*)&b);

    printf("b = [");
    for(int i = 0; i < N; ++i) {
            printf("%3d ", b[i]);
    }
    printf("]\n");

    return 0;
}

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