How can I convert Arduino String to C string type?

Question

I got a String which as I understand is an Arduino object, and got some C++ code:

#include <Wire.h> 
#include <LiquidCrystal_I2C.h>
#include <string.h>
LiquidCrystal_I2C lcd(0x20,16,2); 
boolean borrar = false;
String IP;
void setup()
{
 lcd.init();
 lcd.backlight();
 pinMode(13,OUTPUT);
 Serial.begin(9600);
 Serial1.begin(9600);
}
void loop() {
 while (Serial1.available()) { 
 char caracter = Serial1.read(); //Comprobamos el caracter 
 switch(caracter) {
 default:
 if (borrar) { 
 IP = ""; 
 lcd.clear();
 }
 lcd.print(caracter);
 delay(125);
 borrar = false;
 IP.concat(caracter);
 break; 
 case '\r': 
 case 0x0F:
 case 0x0A: 
 String res = "";
 borrar = true;
 int num= atoi(IP.c_str());
 if (num < 127) 
 res="Clase A";
 if (num == 127) 
 res="Direccion reservada";
 if (num > 127 && num < 192)
 res="Clase B ";
 if (num >= 192 && num < 224)
 res="Clase C ";
 if (num >= 224 && num < 240)
 res="Clase D ";
 if (num >= 240 && num < 255)
 res="Clase E ";
 break; 
 } //fin switch
}//serial disponible
}//fin programa

However, this won't compile because of this line:

int num= atoi(IP.c_str())

As IP is a String and such method works for string. How can I make it compatible (convert it)?

Answer 1

Your code could be improved by removing the use of IP string altogether, and directly calculating its numeric value while characters come in through Serial1:

...
boolean borrar = false;
int IP = 0;
...
void loop() {
 while (Serial1.available()) { 
 char caracter = Serial1.read(); //Comprobamos el caracter 
 switch(caracter) {
 // NOTE it is better to replace default by the list of all digits... 
 case '0':
 case '1':
 case '2':
 case '3':
 case '4':
 case '5':
 case '6':
 case '7':
 case '8':
 case '9':
 if (borrar) { 
 IP = 0; 
 lcd.clear();
 }
 lcd.print(caracter);
 delay(125);
 borrar = false;
 IP *= 10;
 IP += (int) (caracter - '0');
 break; 
 case '\r': 
 case 0x0F:
 case 0x0A: 
 String res = "";
 borrar = true;
 int num= IP;
 if (num < 127) 
 res="Clase A";
 if (num == 127) 
 res="Direccion reservada";
 if (num > 127 && num < 192)
 res="Clase B ";
 if (num >= 192 && num < 224)
 res="Clase C ";
 if (num >= 224 && num < 240)
 res="Clase D ";
 if (num >= 240 && num < 255)
 res="Clase E ";
 break; 
 } //fin switch
}//serial disponible
}//fin programa

This way would bring you 2 advantages:

1. a bit faster than working with String
2. no dynamic memory allocation/deallocation (String does a lot of these) which might lead your program to heap fragmentation and eventually crash.

Note that I have not further refactored your code as I guessed it was just a snippet, not the complete code for your program. Otherwise, I would have performed further refinement like:

• remove num variable since it is the same as IP now
• replace res from String to const char* (to further reduce heap fragmentation due to String usage)

Answer 2

Try

unsigned char z[100];
IP.getBytes(z, 100);
z[IP.length()] = 0;
int n = atoi(z);

To retrieve bytes inside the string. This assumes IP string's length is shorter than 100.

• diegoaguilar: I tried and got: BluetoothLCD.cpp:51:35: error: invalid conversion from ‘char*’ to ‘unsigned char*’ BluetoothLCD.cpp:51:35: error: initializing argument 1 of ‘void String::getBytes(unsigned char*, unsigned int, unsigned int) const’
• rmi: I have updated char to unsigned char
• diegoaguilar: Something weird is going on, now I obtain: BluetoothLCD.cpp:53:33: error: invalid conversion from ‘unsigned char*’ to ‘const char*’ BluetoothLCD.cpp:53:33: error: initializing argument 1 of ‘int atoi(const char*)’
• rmi: If it does not support, IP.toCharArray(z, 100); may be used.
• diegoaguilar: How would code look like?

Answer 3

Try updating to the latest version of the Arduino libraries/IDE.

The c_str() method was introduced to the Arduino String class quite recently, I believe. That line of code certainly works fine for me on Arduino IDE 1.0.5.

• diegoaguilar: I'm using Linux IDE how would I do it?
• Peter Bloomfield: @diegoaguilar - I think you'd have to manually update your Arduino core libraries. You can get them from GitHub: github.com/arduino/Arduino
• Craig: Version 1.0.5 has c_str() it can be downloaded for linux from the official Arduino site: arduino.cc/en/Main/Software