I am working on a project where I need to split incoming data from serial (time to be exact, so aa:bb:cc) and need to split it into aa then bb and cc, ideally into an array. I know I could use readStringUntil and make 3 separate variables, but I would prefer it is in arrays. Here is the code that supposed to do this. It works with predefined string (eg. 00:00:00), but with reading serial it says that the initializer fails to determine size of "input". Thank you in advance for helpful suggestions.

Here is the code:

void setup() { 
  FastLED.addLeds<NEOPIXEL, DATA_PIN>(leds, NUM_LEDS);
  while(!Serial.available()){} //wait until Serial data received
  String vst = Serial.readString();
  char buf[vst.length()+1];
  vst.toCharArray(buf, vst.length());
  char input[] = vst;
  char delic[] = ":";
  char* sub = strtok(input, delic);
  while(sub != NULL){
    sub = strtok(NULL, delic);


  • Instead of vst.toCharArray() (which copies the data) just directly use vst.c_str() which is a char array already. You don't need buf
    – Majenko
    Jan 18, 2019 at 22:04

1 Answer 1


Your main problem is this:

char buf[vst.length()+1];

You can't initialize an array with the return value of a function.

Instead you would need to use alloca to perform the same function:

char *buf = alloca(vst.length()+1);

However, the way you're doing it is wasteful. You don't need buf at all. You can directly access and manipulate the internal char buffer inside the string using vst.c_str();.

For convenience you can point buf at it:

char *buf = vst.c_str();

To complete the picture here's a routine that takes the string and gives you three integers. Since we expect a specific format with only 3 sections to it we don't really need a loop.

// Use readStringUntil to avoid the 1 second delay.
String vst = Serial.readStringUntil('\n');
char *buf = vst.c_str();

char *hourString = strtok(buf, ":");
char *minuteString = strtok(NULL, ":");
char *secondString = strtok(NULL, ":");

if (secondString != NULL) { // Will only be true if we have two colons
    int hours = atoi(hourString);
    int minutes = atoi(minuteString);
    int seconds = atoi(secondString);

    Serial.print("It is ");
    Serial.print(" and ");
    Serial.print(" seconds past ");
} else {
    Serial.println("Invalid time format");

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