Weird Output When Filling Large Array with Random Numbers in C

I'm currently trying to use threads to get the max and min of a large array (only have max right now). I'm have an issue with my output though. When I run my program, I get a HUGE number, even after putting a range for the random number. I'm getting numbers like 2147483644 and 2147483606. I would like it to be a range from 0/1 to 1000, but I just don't seem to be getting the right output. Any ideas?

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

// Size of array 
#define size 100000000 

// Max number of thread 
#define Th_max 4 

// Array 
int a[size];

// Array to store max of threads 
int max_num[Th_max] = { 0 }; 
int thread_no = 0; 

// Function to find maximum 
void* maximum(void* arg) 
{ 

    int i, num = thread_no++; 
    int maxs = 0; 

    for (i = num * (size / 4); i < (num + 1) * (size / 4); i++) { 
        if (a[i] > maxs) 
            maxs = a[i]; 
    } 

    max_num[num] = maxs; 
} 

void fillArray()
{
    for (int i = 0; i < size; i++)
    {
        a[i] = rand() % 1000 + 1;
    }

}

// Driver code 
int main() 
{ 
    int maxs = 0; 
    int i; 
    pthread_t threads[Th_max];


    fillArray();

    for(int i = 0; i < 100000000; i++)
    {
    a[i] = rand();
    }

    // creating 4 threads 
    for (i = 0; i < Th_max; i++) 
        pthread_create(&threads[i], NULL, 
                       maximum, (void*)NULL); 

    // joining 4 threads i.e. waiting for 
    // all 4 threads to complete 
    for (i = 0; i < Th_max; i++) 
        pthread_join(threads[i], NULL); 

    // Finding max element in an array 
    // by individual threads 
    for (i = 0; i < Th_max; i++) { 
        if (max_num[i] > maxs) 
            maxs = max_num[i]; 
    } 

    printf("Maximun Element is : %d", maxs); 

    return 0; 
} 

Thank you.