Prime number calculation in c.

prime.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>

#define sqr(x)      ((x) * (x))

int main(int argc, char *argv[]) {
    unsigned long countNum, workNum, max, total = 0;
    
    // Check argument
    switch (argc) {
        case 1:
            max = 1000000;
            break;
        case 2:
            max = atol(argv[1]);
            if (max >= 2 && max <= 10000000) { // logical max 4294967295
                break;
            }
        default:
            fprintf(stderr, "Invalid argument\n" );
            return 1;
            break;
    }

    struct timeval startTime, endCalcTime, endTime;
    gettimeofday(&startTime, NULL);

    unsigned long *prime;
    prime = malloc(sizeof(long) * max);
    if (prime == NULL) {
        fprintf(stderr, "Error:out of memory");
        return 1;
    }

    *(prime + total++) = 2;
    *(prime + total++) = countNum = 3;

    while (countNum += 2, total < max) {
        workNum = 0;
        while (workNum++, sqr(*(prime + workNum)) <= countNum) {
            if (countNum % *(prime + workNum) == 0) {
                break; /* not prime */
            }
        }
        if (sqr(*(prime + workNum)) > countNum) {
            *(prime + total++) = countNum;
        }
    }

    gettimeofday(&endCalcTime, NULL);
    
    workNum = 0;
    while (workNum < total) {
        if(workNum % 10 == 0) {
            printf("\n");
        }
        printf("%10lu ", *(prime + workNum++));
    }
    printf("\nprime count = %lu\n", total);

    gettimeofday(&endTime, NULL);
    
    printf("calc elapsed time = %lf\n", (endCalcTime.tv_sec - startTime.tv_sec) + (endCalcTime.tv_usec - startTime.tv_usec) * 1.0E-6);
    printf("view elapsed time = %lf\n", (endTime.tv_sec - endCalcTime.tv_sec) + (endTime.tv_usec - endCalcTime.tv_usec) * 1.0E-6);

    free(prime);

    return 0;
}