if statment and garbage value in C

Question

I am newbie to the C programming.I know this is a very simple question but I need some advice. I am exercising with the control structure if statement. I came across one example

Here is the code:-

#include<stdio.h>
int main()
{
    int a = 200, b, c ;
    if (a >= 300)
    {
        b = 100 ;
        c = 200 ;
    }
    printf ( "b=%d\nc=%d", b, c ) ;
    return 0;
}

And the output of this is :-

b=32767
c=0

Here i am expecting the output as the both value to be zero. By seeing this output i am little surprised that why the garbage value in the variable b.

What is the reason behind that?Or I am declaring the variable in wrong way. what should the whole scenario behind this?

If it was always 0 it wouldn't be garbage now would it? – user253751 Jan 11 '16 at 07:07 — user253751, Jan 11 '16 at 07:07

Sourav Ghosh · Accepted Answer · 2016-01-11T07:53:17.830

7

In case

if (a >= 300)

fails, b and c will not be assigned any values.

Now, b and c being automatic local variables, unless initialized explicitly, they contain indeterminate values.

Referring C11, chapter §6.7.9, Initialization

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.[...]

Using unitialized local variables to access their values invoke undefined behavior.

Referring Annex §J.2 of the same standard, for Undefined behavior

The value of an object with automatic storage duration is used while it is indeterminate.

So, it's always advisable to initialize your local variables, like

int a = 200, b = 0, c = 0;

That said, FWIW, int main() should be int main(void) at least to conform to the standard.

edited Jan 11 '16 at 07:53

answered Jan 11 '16 at 07:05

Sourav Ghosh

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But how the variable `c` has `zero` value..I guess it has to be also garbage value.. – Awanti Jan 11 '16 at 07:13
It has a garbage value. It happens to be zero. (The proper term for garbage is "undefined behaviour", which means anything can happen. There's no guarantee that undefined values look like garbage.) – M Oehm Jan 11 '16 at 07:16
That's the thing about undefined behavior, it can work and it can malfunction. – NadavL Jan 11 '16 at 07:18
Yeah first we need to properly initialize the variable to get the desired result..lesson learnt. Thanks a lot. – Awanti Jan 11 '16 at 07:26

score 1 · Answer 2 · answered Jan 11 '16 at 07:17

It seems like you assume declaring a variable implicitly sets it's value to 0.

Declaring a variable but not initializing is a great place for undefined behaviors.

Once you declare a variable, it gets memory allocated to it on the stack, but nothing is done to that memory space, so it can hold the data remains of previous calls, or just garbage data on the stack space.

It's called undefined behavior because that data can incidentally be the actual data you expect, but it can (and probably will) be something else entirely. this is a non-deterministic behavior.

What you need to do is initialize the values first, to make sure your run is deterministic.

score 0 · Answer 3 · answered Jan 11 '16 at 07:06

You have to initialize your variables to 0:

#include<stdio.h>
int main()
{
    int a = 200, b = 0, c  = 0;
    if (a >= 300)
    {
        b = 100 ;
        c = 200 ;
    }
    printf ( "b=%d\nc=%d", b, c ) ;
    return 0;
}

In C and C++ your declaration does not initialize your int variables to 0. Any declaration ends up with random values inside the variable...

if statment and garbage value in C

3 Answers3

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