AVR Freaks

Hello all,

Please I need your Help, because I am a beginner in C and also didn't get it in the Arduino forum.

I am trying to control a GSM modem to send and receive SMS's to read a temperature sensor and switch ON or OFF a motor. I send the AT commands to the modem and need to look for the answer, as well as, read SMS's the split the information.

I already read books about C trying to understand Pointers and Vectors, but, I am a lot confused ( vectors do not need pointers because the name is already pointing to the [0] of the vector, etc.) and I do not find any example similar to what i am using.

At the moment I sent AT command to the modem, read the response but I can not compare it.

Again your help is appreciated because I spend already a week looking for a solution. 

Thanks in advance.

Manuel 

below is a summary of the code used:

I get the following error: undefined reference to 'strstr (  char *, const char * )'

#include <SoftwareSerial.h>
#include <string.h> 


boolean newData = false;

#define rx_pin 2
#define tx_pin 3
const byte num_chars = 100;
char receivedchars [num_chars]; 
unsigned long timeout;
byte rx_ret =0;

char * strstr (  char *, const char * );

char *OK;
char ok_str[]={"OK\n\r"};

SoftwareSerial mySerial(rx_pin, tx_pin);  //rx, tx

byte power= 8;
//byte rx_finished = 0;

void setup()  {
  // define pin modes for tx, rx:
  pinMode(rx_pin, INPUT);
  pinMode(tx_pin, OUTPUT);
  pinMode(power, OUTPUT);
  // set the data rate for the SoftwareSerial port
  mySerial.begin(9600);
  Serial.begin(9600);
  delay(200);
  Serial.println("<Arduino is ready>");
  digitalWrite(power, HIGH);
  delay (2000);
  digitalWrite(power, LOW);
  delay(500);
  mySerial.flush(); // erase

}

void loop()
{
  boolean newData = false;



  mySerial.println("AT");
  rx_ret = read_input_data(175);
  if(rx_ret){
    Serial.println("was here");
  }

  OK = strstr (receivedchars, ok_str);
  if(OK ){
    Serial.println("OK received");
  }

 showNewData();
  
  while(1){ // to avoid keeping senn the command continuesly
  }
}



byte read_input_data(int timeout)
{
  unsigned long prev_time = millis();
  byte num_bytes = 0;
  byte i =0;
  //delay(timeout);
  byte rx_state = 0;

  while((mySerial.available()) || ((unsigned long)(millis()- prev_time) <= timeout)){

    num_bytes = mySerial.available();
    //Serial.println(num_bytes);   
    while(num_bytes)
    {
      num_bytes--;
      if(i <  num_chars){
        receivedchars [i] = mySerial.read();
        i++;
        receivedchars [i] = '\0'; //end the string with null char        
      }
      else
      {
        mySerial.read();
        newData = true;
      }
    }
    
  }
  rx_state = 1;
  return (rx_state); 
} 


// this routine is only to debug porposes

void showNewData() {
  if (newData = true){
    Serial.println(receivedchars);
    receivedchars[0]='\0';
    newData = false;
  }
}

What