overflow issues in implict conversions?

Question

In my machine size of integer data type is 4 bytes so maximum value to the positive side is 2147483647 and to the negative side is -2147483648 in case of a signed int. consider the below c program

#include<stdio.h>

int main(void)
{
    int a = 2147483648;
    int  b = -2147483649;

    printf("%d",a);
    printf("\n%d",b);



    return 0;
}

output: -2147483647 2147483647

why a is implicitly getting converted to -2147483648 and b implicitly getting converted to 2147483647? and why I am getting only for line number 2 the below- given warning

"overflow in implicit constant conversion(-woverflow)"

This can be explained by understanding how 2's complement encoding works. https://stackoverflow.com/questions/1049722/what-is-2s-complement — dernst, Sep 12 '18 at 01:28
1. integer overflow is undefined behavior. You should not expect consistent results from these assignments. 2. Your results 1&2 seem intuitive to me (if I were going to define the desired behavior, that's what it would be). I'm not sure about the missing warning, though. That seems *undesirable* :) — zzxyz, Sep 12 '18 at 01:29
Possible duplicate of [Why does the negation of the minimum possible integer yield itself?](https://stackoverflow.com/questions/45089640/why-does-the-negation-of-the-minimum-possible-integer-yield-itself) — phuclv, Sep 12 '18 at 03:11
@zzyzx There's no UB in this code. You should expect consistent results if rerunning the code on the same implementation. — M.M, Sep 12 '18 at 03:20

score 1 · Answer 1 · answered Sep 12 '18 at 01:39

1

The warning you want is enabled in GCC by -pedantic. In Code Blocks go over to Settings, Compiler and find "Enable warnings demanded by strict ISO C". Turn that on.

answered Sep 12 '18 at 01:39

Zan Lynx

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It drives me nuts what C compilers consider "pedantic" in terms of warnings. – zzxyz Sep 12 '18 at 01:42

score 0 · Answer 2 · answered Sep 12 '18 at 04:20

What is happening in your case is integer overflow which causes undefined behaviour.

From C Committee draft (N1570)

3.4.3
1 undefined behaviour
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements.
2 NOTE: Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).
3 EXAMPLE: An example of undefined behavior is the behavior on integer overflow.

overflow issues in implict conversions?

2 Answers2