Question

I have array from serial read, named sensor_buffer. It contains 21 bytes.

gyro_out_X=((sensor_buffer[1]<<8)+sensor_buffer[2]);
gyro_out_Y=((sensor_buffer[3]<<8)+sensor_buffer[4]);
gyro_out_Z=((sensor_buffer[5]<<8)+sensor_buffer[6]);
acc_out_X=((sensor_buffer[7]<<8)+sensor_buffer[8]);
acc_out_Y=((sensor_buffer[9]<<8)+sensor_buffer[10]);
acc_out_Z=((sensor_buffer[11]<<8)+sensor_buffer[12]);
HMC_xo=((sensor_buffer[13]<<8)+sensor_buffer[14]);
HMC_yo=((sensor_buffer[15]<<8)+sensor_buffer[16]);
HMC_zo=((sensor_buffer[17]<<8)+sensor_buffer[18]);
adc_pressure=(((long)sensor_buffer[19]<<16)+(sensor_buffer[20]<<8)+sensor_buffer[21]);

What does this line do:

variable = (array_var<<8) + next_array_var

What effect does it have on the 8 bits?

<<8  ?

UPDATE:
Any example in another language (java, processing)?

Example for processing: (why use H like header?).

/*
 * ReceiveBinaryData_P
 *
 * portIndex must be set to the port connected to the Arduino
 */
import processing.serial.*;

Serial myPort;        // Create object from Serial class
short portIndex = 1;  // select the com port, 0 is the first port

char HEADER = 'H';
int value1, value2;         // Data received from the serial port

void setup()
{
  size(600, 600);
  // Open whatever serial port is connected to Arduino.
  String portName = Serial.list()[portIndex];
  println(Serial.list());
  println(" Connecting to -> " + Serial.list()[portIndex]);
  myPort = new Serial(this, portName, 9600);
}

void draw()
{
  // read the header and two binary *(16 bit) integers:
  if ( myPort.available() >= 5)  // If at least 5 bytes are available,
  {
    if( myPort.read() == HEADER) // is this the header
    {
      value1 = myPort.read();                 // read the least significant byte
      value1 =  myPort.read() * 256 + value1; // add the most significant byte

      value2 = myPort.read();                 // read the least significant byte
      value2 =  myPort.read() * 256 + value2; // add the most significant byte

      println("Message received: " + value1 + "," + value2);
    }
  }
  background(255);             // Set background to white
  fill(0);                     // set fill to black
// draw rectangle with coordinates based on the integers received from Arduino
  rect(0, 0, value1,value2);
}

Answer 1

Your code has the same pattern:

value = (partial_value << 8) | (other_partial_value)

Your array has data stored in 8 bit bytes, but the values are in 16 bit bytes. Each of your data points are two bytes, with the most significant byte stored first in your array. This pattern simply builds the full 16 bit value by shifting the most significant byte 8 bits to the left, then OR’ing the least significant byte into the lower 8 bits.