Arduino Strings Are BIG!

Interesting test: I was wondering if you could avoid the automatic string copy to RAM by putting strings inside a subroutine:

 
#include <avr/pgmspace.h>
int freeRam () {
  extern int __heap_start, *__brkval;
  int v;
  return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);
}
void sub(void)
{
  char *text="ssssssssssssssssssssssssssssssss";
  Serial.print("inside function: ");
  Serial.println(freeRam());
//  text[0]='\0';
}
void setup() 
{
  Serial.begin(9600);
  Serial.println(freeRam());
  sub();
  Serial.println(freeRam());
}
void loop() 
{
}

Run it, and you’ll get a memory display of RAM before, during, and after the subroutine call, something like this:

1840
inside function: 1834
1840

But uncomment the line

//  text[0]='\0';

and run again – likely, you’ll get something more like this:

1806
inside function: 1800
1806

What happened? In the first one, the compiler optimized the text string right out of the system – until you did something with it (like setting text[0]) then it put it back in, and with a vengeance: that string is now in free memory RAM from the start of the program (notice the value before the subroutine was called – no change after).

What does this mean? If you use strings, be careful of them – anywhere they exist they add to RAM usage. Of course, if you move your strings to program memory, that’s another matter…

SOME ITEMS YOU MIGHT ENJOY
Arduino IIC/I2C/TWI 162 1602 16X2 Serial Blue LCD Module Display Screen
$3.74
End Date: Tuesday Nov-18-2014 18:03:51 PST
Buy It Now for only: $3.74
Buy It Now | Add to watch list
Version Board Free USB Cable For Arduino UNO R3 ATmega328P ATmega16U2
$7.87
End Date: Tuesday Nov-18-2014 16:22:32 PST
Buy It Now for only: $7.87
Buy It Now | Add to watch list
GOOD 5V Eight Channel Relay Module For PIC AVR DSP ARM Arduino
$1.25 (2 Bids)
End Date: Monday Oct-20-2014 15:03:06 PDT
Bid now | Add to watch list


2 thoughts on “Arduino Strings Are BIG!

  1. Nice tip – thanks, man! This is an example of tweaky-technical information conveyed to web-visitors done *RIGHT*. Straight-forward, concise, complete. Good job.

  2. If you’re worried about ram space, you should use the “const” statement (http://www.arduino.cc/en/Reference/Const) who is used to load the variable in ROM instead of RAM. Using const means that you’re not going to change the value during running time, but when dealing with menus in LCD displays or other constants its very useful.