Function:

#include 
#include 
#include 
#include 

char *password_generator(const int pwd_len, const char *char_array) {
    int i;
    int n;
    int r;
    int sizeof_array;
    char c;
    char *password;
    static int seeded = 0;

    if (!char_array) { // If hasn't a char_array set a default
        char_array = "abcdefghijklmnopqrstuvwxyz" \
                        "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    }
    sizeof_array = strlen(char_array);

    if(!seeded) { // Check if already has a seed 
        srand((unsigned int)time(0)); // Seed number for rand()
        seeded = 1;
    }
    // Alloc the memory to the password
    password = (char *) malloc(sizeof(char) * (pwd_len+1));

    for (i = 0; i < pwd_len; i++) {
        r = rand(); // Get a random number
        // using mod to reduce the number to sizeof_array in max 
        // (Ex: 3452354 % 34: 28) 
        n = r % sizeof_array;
        c = char_array[n]; // Get the char in the 'n' pos
        password[i] = c;
    }
    password[i] = '\0';
    return password;
}

How to use it?!

int main(void) {
    char *password;
        
    password = password_generator(12, "0123456789");
    printf("Password: [%s]\n", password);
    free(password);
        
    password = password_generator(10, NULL);
    printf("Password: [%s]\n", password);
    free(password);

    return 0;
}