Optimised random alphanumeric string generator

Question

I'm trying to build arduino nonce generator, but the only thing I found is this question on arduino forum but I can't find out how to make it work for me. I checked and Serial.available() is always 0 for me and if I delete the If I still get nothing

My code:

void setup() {
 Serial.begin(9600);
 randomSeed(analogRead(0));
}
void loop() {
 byte randomValue;
 char temp[5];
 char letter;
 char msg[50]; // Keep in mind SRAM limits
 int numBytes = 0;
 int i;
 int charsRead;
 
 if(Serial.available() == 0) 
 {
 charsRead = Serial.readBytesUntil('\n', temp, sizeof(temp) - 1); // Look for newline or max of 4 chars
 temp[charsRead] = '0円'; // Now it's a string
 numBytes = atoi(temp);
 if(numBytes > 0)
 {
 memset(msg, 0, sizeof(msg));
 for(i = 0; i < numBytes; i++) {
 randomValue = random(0, 36);
 msg[i] = randomValue + 'a';
 if(randomValue > 26) {
 msg[i] = (randomValue - 26) + '0';
 }
 }
 Serial.println("Here is your random string: ");
 Serial.println(msg);
 Serial.print("I received: ");
 Serial.println(numBytes);
 delay(1000);
 }
 }
}

How do I make it work?

Do you have some working random alphanumeric string generator that you are using?

• chrisl: Why do you check for if(Serial.available() == 0)? If you want to read data from Serial you should check for if(Serial.available() > 0), which can be written shorter as if(Serial.available()). Please try that
• Nick Gammon: Is this right? You are reading in a 4-digit number, and then using that as the number of digits to put random characters into a 50-byte array? Why not read a 2-digit number? Or at least test if the number is > 49 or not.
• Edgar Bonet: if(randomValue > 26) should be if(randomValue >= 26). Other than the fixes suggested in these three comments, the sketch should work as advertised.
• krystof18: @chrisl even with that, the code never goes into that if

Answer 1

Your problem seems to be related to how you get the actual length you want to generate. There are other ways you could do that. Since your max seems to be 4 characters, you could possibly enter "9999" which would overflow the 50 bytes temp variable.

You could simply read until \n and then atoi. Then cap at 50.

As a side note, here's a function I've used in the past to generate (pseudo)random strings for network requests (e.g. state or nonce parameters)

#define MAX_UID 8 /* Change to whatever length you need */
const char * generateUID(){
 /* Change to allowable characters */
 const char possible[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
 static char uid[MAX_UID + 1];
 for(int p = 0, i = 0; i < MAX_UID; i++){
 int r = random(0, strlen(possible));
 uid[p++] = possible[r];
 }
 uid[MAX_UID] = '0円';
 return uid;
}