#include <iostream>
#include <time.h>

int main(void){
    time_t seconds;
    time(&seconds);
    srand((unsigned int) seconds);
    unsigned int i,j;

    start:
    std::cout<<"Enter amount of chars (max 16386): ";
    std::cin>>j;
    if (j>16386)
    {
        std::cout<<"error: too much\n";
        goto start;
    }
    for (i=0;i<1;++i)
    {
        unsigned int a[j];
        for (i=0;i<j;++i)
        {
            while (!(((a[i]=rand())>='0'&&a[i]<='9')||(a[i]>='A'&&a[i]<='Z')||(a[i]>='a'&&a[i]<='z')))
                ;
            printf("%c",a);
        }
    }
    printf("\n");
    std::cout << "Press enter to continue...";
    std::cin.sync();
    std::cin.get();
    std::cout << std::endl;
    goto start;
}

Code here