How to Generate a Random Alpha-Numeric String of Specified Length in C ?

Creating a Random Alpha-Numeric String in C

This article addresses the query of how to generate a random string comprising alphanumeric characters of a specified length in C .

The solution presented by Mehrdad Afshari is effective, but for this basic task, it may be somewhat verbose. Lookup tables can provide a more concise approach:

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

std::string gen_random(const int len) {
    static const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";
    std::string tmp_s;
    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[]) {
    srand((unsigned)time(NULL) * getpid());     
    std::cout << gen_random(12) << "\n";        
    return 0;
}

It is important to note that the rand function generates pseudo-random numbers, which may not be of high quality. For more secure applications, consider using a cryptographically strong random number generator (CSPRNG) instead.

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