Problem with atoi in 0021

i'm trying convert sketch 0018 0021. 0021 has built in string function replacing wstring library. compile error @ n = atoi(readstring); line in code below.  anybody know changed cause compile error?

code: [select]


// zoomkat 7-30-10 serial servo test
// type servo position 0 180 in serial monitor
// writemicroseconds, use value 1500
// http://arduino.cc/en/uploads/tutorial/string.zip wstring.h

//#include <wstring.h> //provides easy string handling
string readstring = string(100);
#include <servo.h>
servo myservo;  // create servo object control servo

void setup() {
     serial.begin(9600);
       myservo.attach(9);
       }

void loop() {

       while (serial.available()) {
       delay(10);  
         if (serial.available() >0) {
       char c = serial.read();  //gets 1 byte serial buffer
       readstring += c;}
     //readstring.append(c); } //makes string readstring
       }
       
     if (readstring.length() >0) {
     serial.println(readstring);
     int n;
     n = atoi(readstring); //convert string number
     //myservo.writemicroseconds(n);
     myservo.write(n);
     readstring="";
     }
  }



try:

code: [select]

n = atoi(readstring.getbytes());

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