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cintegercomparisonlong-integer

arithmetic of long int in c

I have the following source compiled in Linux 3.10.0-957.5.1.el7.x86_64,g++ version 4.8.5

Case1:

printf("INT_MAX=(%d) , INT_MIN=(%d) \n",INT_MAX,INT_MIN);
int ix= 500  ;
long int lx1=0,lx2=0;
lx1=2147483647 + 10 ;
lx2=2100000000 ;
if( ix < (lx1-lx2) )
    printf("ix is not bigger \n");
else
    printf("ix is bigger \n");

compiled with warning :

warning: integer overflow in expression [-Woverflow]
lx1=2147483647 + 10 ;

and output :

INT_MAX=(2147483647) , INT_MIN=(-2147483648) 
ix is bigger

and the following source Case2 :

printf("INT_MAX=(%d) , INT_MIN=(%d) \n",INT_MAX,INT_MIN);
int ix= 500  ;
long int lx1=0,lx2=0;
lx1=2200000000 + 10 ;
lx2=2100000000 ;
if( ix < (lx1-lx2) )
    printf("ix is not bigger \n");
else
    printf("ix is bigger \n");

compiled without warning and the output :

INT_MAX=(2147483647) , INT_MIN=(-2147483648) 
ix is not bigger

My question : Why Case1 output can be wrong ? lx1 and lx2 both long int, in this box it is size 8 bytes , how come 2200000000 is fine but 2147483647 is not for lx1 ?!

Comparing int with long and others is referd , still can not figure it out .

Solution

  • The calculation of 2147483647 + 10 happens with int datatypes, because both values fit into an int. The result overflows and then the result will be extended to a long, but that's too late.
    Suffix the number with an l to make it a long: 2147483647l + 10.

    2200000000 is too big for an int, therefore it is a long, so the additon works as expected.

    https://ideone.com/ClJI8F