Generate multiple random alphanumeric in C++ [duplicate]

I want to generate multiple random alphanumeric string within one second, but my code always gives the same string, I understand that it is because the time () (typically) has a resolution of 1 second. Is there any way to solve this?

Here is my code:

#include <iostream>
#include <ctime>
#include <unistd.h>

using namespace std;

string gen_random(const int len) {

    string tmp_s;
    static const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";

    srand( (unsigned) time(NULL) * getpid());

    tmp_s.reserve(len);

    for (int i = 0; i < len; ++i)
        tmp_s += alphanum[rand() % (sizeof(alphanum) - 1)];

    return tmp_s;

}

int main(int argc, char *argv[]) {

    cout << gen_random(4) << " and " << gen_random(5) << endl;

    return 0;
}