serial.c

char inData[256]; // Allocate some space for the string
char inChar = -1; // Where to store the character read
byte index = 0; // Index into array; where to store the character

void read_uart() {
    while (Serial.available() > 0)  {
        if (index < 256) {
            inChar = Serial.read(); // Read a character
            inData[index] = inChar; // Store it
            index++; // Increment where to write next
            inData[index] = '\0'; // Null terminate the string
        }
    }

    if (strcmp(inData, "") != 0) {
        Serial.println(inData);

        for (int i = 0; i < 256; i++) {
            inData[i] = 0;
        }
        index = 0;
        return;
    }
    return;
}

void setup() {
    Serial.begin(9600);
    Serial.write("Power On");
}

void loop()
{
    read_uart();
}