Integer value promotion to unsigned int vs signed int in c

Asked Aug 20 '19 at 19:17

Active Aug 20 '19 at 22:46

Viewed 77 times

I have two c codes, the difference between them is that one uses char and one uses int, as follows,

First:

if case is true

int x = -1;
unsigned int y = 2;
if(x>y)

Second:

the if case is false

char x = -1;
unsigned char y = 2;
if(x>y)

Why does this happen?

edited Aug 20 '19 at 22:46

EsmaeelE

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asked Aug 20 '19 at 19:17

Ahmad Anwar

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be aware that the signedness of `char` is implementation-defined. – Christian Gibbons Aug 20 '19 at 19:20
@Barmar the dup is about comparing for equality. – Weather Vane Aug 20 '19 at 19:30
Dupe looks like it is C++ even though there is also a C tag. – Christian Gibbons Aug 20 '19 at 19:32
@WeatherVane I think the promotion rules are the same for equality and inequality tests. – Barmar Aug 20 '19 at 19:34
@ChristianGibbons C and C++ are similar in this area. – Barmar Aug 20 '19 at 19:34
Basically everything is first expanded to int if both ranges are representable in an int, else unsigned int. In the first case both are converted to unsigned int, in the second to int – Antti Haapala -- Слава Україні Aug 20 '19 at 19:36
In the second case, both values can be promoted to `int` without loss so the comparison works correctly. In the first case, they cannot so are converted to `unsigned int`, and the negative value is converted by the implementation to a large positive value. C11 **§6.3.1.1.2** says *If an `int` can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an `int`; otherwise, it is converted to an `unsigned int`. These are called the **integer promotions**. All other types are unchanged by the integer promotions.* – Weather Vane Aug 20 '19 at 19:37
@Barmar I thought there was a preference to separate C and C++ questions to fight the narrative that "C/C++" is a language and that we want them to be treated separately to avoid the issues that come along with assuming what is true with one is true of another. – Christian Gibbons Aug 20 '19 at 19:37
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@Barmar but it does not answer this question: why the comparison breaks. – Weather Vane Aug 20 '19 at 19:38
@WeatherVane this one does – Antti Haapala -- Слава Україні Aug 20 '19 at 19:40
@AnttiHaapala yes is more relevant. – Weather Vane Aug 20 '19 at 19:41
@WeatherVane if you do have sth to add then please do edit Lundin's answer or add another one ;) – Antti Haapala -- Слава Україні Aug 20 '19 at 19:43
@AnttiHaapala why would I do that to an exemplary answer? I posted my comment before you added the dupe (and before I saw your earlier comment - I was typing). – Weather Vane Aug 20 '19 at 19:51
@WeatherVane well it might be sth that the exemplary answer is missing :d – Antti Haapala -- Слава Україні Aug 20 '19 at 19:53

Integer value promotion to unsigned int vs signed int in c

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