Why is abs(0x80000000) == 0x80000000?

c integer bit-manipulation 32-bit
sigjuice picture sigjuice · Mar 29, 2010 · Viewed 8.2k times · Source

I just started reading Hacker's Delight and it defines abs(-231) as -231. Why is that?

I tried printf("%x", abs(0x80000000)) on a few different systems and I get back 0x80000000 on all of them.

Answer

James McNellis picture James McNellis · Mar 29, 2010

Actually, in C, the behavior is undefined. From the C99 standard, §7.20.6.1/2:

The abs, labs, and llabs functions compute the absolute value of an integer j. If the result cannot be represented, the behavior is undefined.

and its footnote:

The absolute value of the most negative number cannot be represented in two’s complement.

Related questions

Check value of least significant bit (LSB) and most significant bit (MSB) in C/C++

I need to check the value of the least significant bit (LSB) and most significant bit (MSB) of an integer in C/C++. How would I do this?

Read More
Catch and compute overflow during multiplication of two large integers

I am looking for an efficient (optionally standard, elegant and easy to implement) solution to multiply relatively large numbers, and store the result into one or several integers : Let say I have two 64 bits integers declared like this : uint64_t …

Read More
How to convert integer to string in C?

I tried this example: /* itoa example */ #include <stdio.h> #include <stdlib.h> int main () { int i; char buffer [33]; printf ("Enter a number: "); scanf ("%d",&i); itoa (i,buffer,10); printf ("decimal: %s\n",buffer); itoa (i,…

Read More