I Wants To Ask How To Know How Many Bytes Xbee Of Zigbee Has Sent So That I Can Receive On Serial Communication

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C++

Arduino

I have write code as follow

#include <SoftwareSerial.h>
SoftwareSerial softSerial(10, 11); // RX, TX

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  softSerial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
  at_command(01,'S','L',"");
  if(softSerial.available()){
    Serial.print(softSerial.read());
  }
}

// Sending AT command to Connected radio
void at_command(char id, char c1, char c2, char *cmdData) {
  // Start delimiter
  softSerial.write(0x7E);    // [Fixed]
  // paylode length [Depend on length of frame minimum 5]
  softSerial.write((byte)0x00);    // MSB
  softSerial.write(0x05);    // LSB
    // Frame specifid data (Pay load)
    softSerial.write(0x88);  // Frame type [Fixed]
    softSerial.write(id);    // Frame ID [Unique ID to veryfy response. if =0 will not get any response]
    // AT Command
    softSerial.write(c1);    // First Charecter
    softSerial.write(c2);    // Second Charecter
    // Command data
    long cmdDataSum = 0;
    if(sizeof(cmdData) > 0) {
      for(int i=0; i<= sizeof(cmdData)-1; i++) {
        softSerial.write(i);  // [maximum 256 charecters]
        cmdDataSum = cmdDataSum + i;
      }
    } else { // do nothing
      ;
    }
  // Checksum
  long sum = 0x88 + id + c1 + c2 + cmdDataSum;
  softSerial.write(0xFF - (sum - 0xFF));
  // just for a sake of guard
  delay(10);
}

// Receiving AT command response from Radio
void at_response(){
  // Received AT comman responce from Coordinator
  if(Serial.available() > 21 ) {
    if(Serial.read() == 0x7E) {
      for (int i=0; i<19; i++) {
        byte discard = Serial.read();
      }
      byte atResponse = Serial.read();
    }
  }
}

Now suppose if i will send some command to XBee that will return AT Command Response to read which i have to use a loop as i have done in at_response() function along with putting values i am reading as check as in

if(Serial.available() > 21 )

. What if XBee send some variable length response and i have to read that?

I hope question is clear! let me explain once more that i want to read some variable length response through Zigbee on serial as i have done with fixed length response by

if(Serial.available() > 21 )

Can anyone tell me how to do so?