Count fuel liters with Arduino
You have declared count_pulse as a variable and NOT as function prototype.

int pin = 2;
volatile unsigned int pulse;
float volume = 0;
float flow_rate =0;
const int pulses_per_litre = 2000;

void count_pulse(); \\ &lt;--- Function prototype

void setup()
{
    Serial.begin(9600);
    pinMode(pin, INPUT);
    attachInterrupt(0, count_pulse, RISING);
}