What does 1000L mean here? Who can help?

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void playtone(int tone, int duration) {
  (long = 0; < duration * 1000l; += tone * 2) {
    digitalwrite(speakerpin, high);
    delaymicroseconds(tone);
    digitalwrite(speakerpin, low);
    delaymicroseconds(tone);
  }

it means 1000 long integer , in context used make sure multiplication "duration * 1000" results in long integer , not overflow (as without l).


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