SRAM memory usage on Arduino Uno [Solved]

Projects Programming
1

I'm trying to understand how memory is used on the Arduino.
If you look at the web-page http://arduino.cc/en/Tutorial/Memory it states that a declaration like:

char message[] = "I support the Cape Wind project.";

will use SRAM. My program seems to suggest otherwise. The way I am checking free memory came from Arduino Playground - HomePage using:

int freeRam () {
    extern int __heap_start, *__brkval; 
    int v; 
    return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval); 
}

If I allocate arrays, strings, etc, I do not see this reflected in the memory usage.
ie: If I increase/decrease these strings/arrays/etc, there is no change in the free memory.
If I do a malloc(), I do see this change, as expected.

So, my question is, is the Arduino memory web-page incorrect, is the code I'm using above for freeRam() incorrect, or
am I doing something really stupid? Below is my code:

int             freeRam() ;
extern void*    malloc() ;

void setup() {
    char message[] = "I support the Cape Wind project.";

    Serial.begin( 9600 ) ;

    Serial.print( "debug: Free ram = " ) ;
    Serial.println( freeRam() ) ;
    Serial.flush() ;

    char *str = (char *)malloc( 500 ) ;

    Serial.print( "debug: Free ram after malloc = " ) ;
    Serial.println( freeRam() ) ;
    Serial.flush() ;
}

void loop() {
    delay( 1000 ) ;
}

int freeRam () {
    extern int __heap_start, *__brkval; 
    int v; 
    return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval); 
}

If I increase message[], there is no SRAM memory usage change shown.

Possibly related to this, is that if I use Serial.print() to print out strings, I do see the memory being used, however, it is double what I expect. For eg: if I increase a string by 10 bytes, and use Serial.print() to print it out, memory availability will decrease by 20 bytes, even though sizeof() of the string (and of type char) confirms that only 1 byte is being used per character. I assume that this is just because of how Serial.print() is implemented and the lesson here is to be aware that it can use double the memory you may expect.

cheers,
-rj

2

To save that memory use the F macro to put strings that don't change in program memory.

Serial.print(F( "debug: Free ram = " ) );

const char message[] = "I support the Cape Wind project.";

Will save SRAM by putting the string in program memory.

3

If I increase message[], there is no SRAM memory usage change shown.

Because you're not using message[], so the compiler is optimizing it out.

4

Thanks for the replies.

I already knew about the F() macro.
I was trying to understand what was happening without using the F() macro. Thanks!

I had suspected the compiler optimised out the message[] because it was not being used. The original program I submitted was cut down to its most basic short form to make it easier to see what I am doing. A previous attempt tried to force the use of the string by doing a:

char str[] = "This is a big string....1234567890" ;
char ch1   = str[0] ;      // just to make sure str gets used

However, further experimentation now suggests that the compiler saw through the ruse and still did not use either of str[] or ch1 (smarter than I thought :))
Only when I do some I/O does it include it. If I do a:

#define SIZE    200

    int nums[ SIZE ], i ;
    
    for ( i=0 ; i < SIZE ; i++ ) {
        nums[ i ] =+ 1 ;
    }

again to try to force use of the data, that by itself will also be ignored. But as soon as I add a

Serial.println( nums[5] ) ;

then I can see the memory usage change as nums[] is included and as I modify SIZE

Thanks for the help!
-rj