Syntax related query during variable declaration and initialization

Question

Consider the following code snippet

int main(){
 int a, b, c;
 printf("%d %d %d", a, b, c);
 return 0;
}

This is going to display some garbage value.
Now, consider this snippet.

int main(){
 int a, b, c = 0;
 printf("%d %d %d", a, b, c);
 return 0;
}

This displays 0 0 0 for every execution. But I've only initialized variable c to 0. Is this some shorthand for doing something like, int a=0, b=0, c=0;

Don't know how to look for such doubt in official documentation.

The values of a and b are indeterminate in both versions. It's just by accident that they're being set to 0 in the second one. The compiler might be using an optimization when initializing c that happens to fill the whole block of memory with zeroes. — Barmar
– Barmar, Commented Dec 30, 2022 at 2:26
Just because the values are indeterminate it doesn't mean they have to be garbage. They can be anything, including zero.. — Barmar
– Barmar, Commented Dec 30, 2022 at 2:27
@Barmar, that means even though a, b are being initialized to 0, that is still garbage value? — taurus05
– taurus05, Commented Dec 30, 2022 at 2:29
They have to contain something. 0 is just as good as anything else. — Barmar
– Barmar, Commented Dec 30, 2022 at 2:31
To understand what's happening, you should really look at the assembly code. To request the assembly code from gcc, use the -S option. Or better yet, step through the assembly with a debugger. — user3386109
– user3386109, Commented Dec 30, 2022 at 3:13

chux · Accepted Answer · 2022-12-30 12:55:46Z

3

Without initialization, a, b are indeterminate right after int a, b, c = 0;

Their values are neither certainly consistent (they could be random) nor safe to access.

If an initialization is specified for the object, ...; otherwise, the value becomes indeterminate each time the declaration is reached. C17dr §6.2.4 6

indeterminate value
either an unspecified value or a trap representation 3.19.2

unspecified value
valid value of the relevant type where this International Standard imposes no requirements on which value is chosen in any instance 3.19.3

trap representation
an object representation that need not represent a value of the object type 3.19.4

Didn't know how to look for such doubt in official documentation.

See Where do I find the current C or C++ standard documents? and search the spec for initialization.

edited Dec 30, 2022 at 12:55

answered Dec 30, 2022 at 5:21

chux

158k17 gold badges160 silver badges311 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

taurus05 Over a year ago

Thank you very much! Found it very insightful and helpful.

taurus05 · Accepted Answer · 2022-12-31 05:11:15Z

0

Well, a and b are getting garbage on both examples. The only difference is that their garbage are different on each execution. Like, they are put on an address that can contain anything, as, -2130381 or 83943097 or even 0. X% of the times you can get a 0 on an uninitialized value, and it will work on the first Y modifications of the code. After that, your code can just broke and you may spend some time debugging it.

edited Dec 31, 2022 at 5:11

taurus05

2,57420 silver badges31 bronze badges

answered Dec 30, 2022 at 3:02

Bitenco

111 silver badge3 bronze badges

Collectives™ on Stack Overflow

Syntax related query during variable declaration and initialization

2 Answers 2

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related