need to cast operands when doing long math

Using Arduino Programming Questions
1

Hi folks,

I'm doing some long (32 bit) math.

unsigned long int tt = 365 * 24 * 60 * 60 ;
Serial.println(tt) ;

yields 13184, should be 31536000.

With static cast it calculates correctly.
tt = (unsigned long int) 365 * (unsigned long int) 24 * (unsigned long int) 60 * (unsigned long int) 60 ;
Serial.println(tt) ;

seems that the interim math is 16 bit not 32 bit.

I would have thought that gcc would have given me a warning about this. (Are we not running with -Wall? )

This particular problem was easy to find/fix but this sort of problem can make one question the foundations and general slow development down.

Can I pass -Wall (or some other recommended switches) to gcc so that nasties are flagged? or other suggestions?

Thx,
Bruce

-- code --

#include <Time.h>  
#include <stdio.h>

void setup() {
  Serial.begin(9600); 
  unsigned long int  tt =  365 * 24 * 60 * 60 ;
  time_t time  = 365 * 24 * 60 * 60 ;
  Serial.println("Init");
  Serial.println(time) ;
  Serial.println(tt) ;
  tt=60 ;
  Serial.println(tt) ;
  tt*=60 ;
  Serial.println(tt) ;
  tt*=24 ;
  Serial.println(tt) ;
  tt*=365 ;
  Serial.println(tt) ;
  tt =  (unsigned long int) 365 * (unsigned long int) 24 * (unsigned long int) 60 * (unsigned long int) 60 ;
  Serial.println(tt) ;
}

void loop() {
  delay(1000) ;
}

-- serial port --
Init
13184
13184
60
3600
86400
31536000
31536000

2

Instead of unsigned long int tt = 365 * 24 * 60 * 60 ;

try:

unsigned long int tt = 365UL * 24UL * 60UL * 60UL ;

Lefty

3

seems that the interim math is 16 bit not 32 bit

That's because the native "int" for the Arduino is 16 bits.

4

Thanks for the replies!

in 32 bit gcc 4.3.4 I do not get a warning message with the equivalent:

unsigned int tts2 = (unsigned short) 365 * (unsigned short)24 * (unsigned short) 60 * (unsigned short)60 ;

sooo... I guess it's one of those things to remember.

BTW, this does print a nice warning by default ( with 32bit gcc, no -Wall needed) :

unsigned short int tts = 365UL * 24UL * 60UL * 60UL ;
testcase.cpp:6: warning: large integer implicitly truncated to unsigned type

5

bcaldwellma:
BTW, this does print a nice warning by default ( with 32bit gcc, no -Wall needed) :
unsigned short int tts = 365UL * 24UL * 60UL * 60UL ;
testcase.cpp:6: warning: large integer implicitly truncated to unsigned type

That's because a short is always 16 bits, just as a long is always 32 bits (at least on most computers I've encountered).

However, the int is special - it's considered the most convenient size for the processor to use, and that's why it varies so much - some computers use it for 16 bit wide value, some 32.