Skip to content

Instantly share code, notes, and snippets.

@husobee

husobee/arithmetic.c

Created Sep 10, 2017
Embed
What would you like to do?
arith-overflow
#include <stdio.h>
int main() {
// Given an unsigned char, if we add 1 to the max value an unsigned char can have,
// we strangely get the correct value, which should have overflown the char...
unsigned char a = 255;
printf("%lu + 1 == %d \n", a, a + 1);
// Output: 255 + 1 == 256
// When we take this value and assign it to an unsigned char, and look at the
// value again we get ...
unsigned char b = a + 1;
printf("%lu + 1 == %d \n", a, b);
// Output: 255 + 1 == 0
// This is because in C all arithmetic UP CASTS if there is an overflow!
// Looking at the type of the result of a + 1
printf("sizeof a == %d; sizeof a+1 == %d\n", sizeof(a), sizeof(a+1));
// Output: sizeof a == 1; sizeof a+1 == 4
// As you can see the sizeof a is 1 byte, and the sizeof a+1 is 4 bytes
//
unsigned char i = 0;
unsigned char j = 255;
printf(" 0 - 255 == %d\n", i - j);
// Output: 0 - 255 == -255
printf(" 0 - 255 == %d\n", (unsigned char)(i - j));
// Output: 0 - 255 == 1
// You can see that this can get confusing if you do not explicitly cast arithmetic
printf("-128/-1 == %d\n", (signed char)(-128/-1));
// Output: -128/-1 == -128
// WAT? :)
return 0;
}
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment