How to create a dynamically sized global array? - c

Question

I am making a multi-threaded program to find prime numbers in C. How do I take an input in the following C program and not use (#define N =88)?
I am getting the following error:
main.c:8:7: error: expected declaration specifiers or ‘...’ before string constant
scanf("%d", &N);
^~~~
#include <stdio.h>
#include <pthread.h>
#include <conio.h>
#define MAX_THREADS 4
int N;
scanf("%d", &N);
int prime_arr[N]={0};
void *printprime(void *ptr)
{
int j,flag;
int i=(int)(long long int)ptr;
while(i<N)
{
printf("Thread id[%d] checking [%d]\n",pthread_self(),i);
flag=0;
for(j=2;j<=i/2;j++)
{
if(i%j==0)
{
flag=1;
break;
}
}
if(flag==0 && (i>1))
{
prime_arr[i]=1;
}
i+=MAX_THREADS;
}
}
int main()
{
pthread_t tid[MAX_THREADS]={{0}};
int count=0;
for(count=0;count<MAX_THREADS;count++)
{
printf("\r\n CREATING THREADS %d",count);
pthread_create(&tid[count],NULL,printprime,(void*)count);
}
printf("\n");
for(count=0;count<MAX_THREADS;count++)
{
pthread_join(tid[count],NULL);
}
int c=0;
for(count=0;count<N;count++)
if(prime_arr[count]==1)
printf("%d ",count);
return 0;
}

Answer 1

You can't have general statements outside of functions.
The simple solution is to read the input in the main function and then create the array in the main function as well.
And keep the array as a local variable inside the function, and pass it as an argument to the functions to call (together with its size).
To mass multiple arguments to the thread function, create a structure with the "arguments" as members, and pass a pointer to one such structure.