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Writes binary data to the serial port. This data is sent as a byte or series of bytes; to send the characters representing the digits of a number use the print() function instead.


Serial.write(buf, len)


Serial: serial port object. See the list of available serial ports for each board on the Serial main page.
val: a value to send as a single byte.
str: a string to send as a series of bytes.
buf: an array to send as a series of bytes.
len: the number of bytes to be sent from the array.


write() will return the number of bytes written, though reading that number is optional. Data type: size_t.

Example Code

void setup() {

void loop() {
  Serial.write(45); // send a byte with the value 45

  int bytesSent = Serial.write("hello");  //send the string "hello" and return the length of the string.

Notes and Warnings

As of Arduino IDE 1.0, serial transmission is asynchronous. If there is enough empty space in the transmit buffer, Serial.write() will return before any characters are transmitted over serial. If the transmit buffer is full then Serial.write()will block until there is enough space in the buffer. To avoid blocking calls to Serial.write(), you can first check the amount of free space in the transmit buffer using availableForWrite().