Thread: structure and union

My two questions regarding structures and union are:
1. Why does the following program results in segmentation fault.

Code:

#include<stdio.h>
#include<conio.h>
int main(void)
{
	union a
	{
		char ch[2];
		int i;
	};
	union a key;
	key.ch[0]='5';
	printf("%d %d %d",key.ch[0],key.ch[1],key.i);
	getch();
}

2. Why the output of the code 24.

Code:

#include<stdio.h>
#include<conio.h>
int main(void)
{
	struct
	{
		char ch;// char 1 byte
		int i;//int 4 bytes
		float f;//float 4 bytes
		double r;// double 8 bytes
	}v;
	printf("%d",sizeof(v));
	getch();
}

I know slack byte thing but that should also result in 20bytes coz my PC's word size is 4 bytes(32bit OS).
Thanks

I agree with tabstop: First question should not cause a segfault as it is posted. If it does, I suspect you are either not actually running the code you posted [perhaps there is some minor syntax error, and you are running the old version that is different?]

In the second case, you can't really know what the size of a struct will be without going into details of what compiler, what processor, and what settings the compiler is given. Any number from 15 and upwards is possible. In this case, we can infer that double has an alignment equal to it's size [this is very common] and that a 4 byte int will be aligned to 4 bytes [but it's not NECESSARILY always true that this is the case - a 16-bit compiler that uses 32-bit float and 64-bit double could also give the same size, just that there would be more bytes between the int and float element].

Relying on size and offset of structures is very dangerous.

--
Mats

Originally Posted by matsp

I agree with tabstop: First question should not cause a segfault as it is posted. If it does, I suspect you are either not actually running the code you posted [perhaps there is some minor syntax error, and you are running the old version that is different?]
--
Mats

I ran exactly the same code that's why I'm saying that it's producing run time error. The error is

Run-Time Check Failure #3 - The variable 'key' is being used without being defined.

By the way are segfault and run time error two different things? It might be the term 'segfault' that I used in my earlier post which is causing problem to you in understanding. I thought both segfault and run time error are same thing.

> By the way are segfault and run time error two different things? It might be the term 'segfault' that I used in my earlier post which is causing problem to you in understanding.
Yes, VASTLY different things - be specific next time.

Had you posted the actual error message (not your faulty interpretation), we would have been all over it right away and you would have had your answer a day earlier.

A segfault is an access (read or write) to memory which does NOT exist. Accessing NULL is one typical example.

A run-time check is the code examining the program state as it goes. In this case, it looked at the variable in question and determined that you hadn't written to it before reading it.

The words say it all really.
The variable 'key' is being used without being defined.

I will be more careful next time.
But still why it says that the variable 'key' is is not defined. I think I've defined it here

Code:

union a key;

Also when I change this line to

Code:

key.ch[0]='5';// to key.i=5; the code works properly

>> But still why it says that the variable 'key' is is not defined. I think I've defined it here

That code should compile and run fine.

EDIT: What you probably meant anyway was:

Code:

#include<stdio.h>
#include<conio.h>
int main(void)
{
	union a
	{
		char ch[2];
		int i;
	};
	union a key;
	key.ch[0]='5';
	printf("%c %c %d\d",key.ch[0],key.ch[1],key.i);
	getch();
}

> But still why it says that the variable 'key' is is not defined. I think I've defined it here
You only wrote key.ch[0]

key.ch[1] was untouched, and key.i is basically junk (partially initialised by it's overlap with key[0]).

Originally Posted by Salem

> But still why it says that the variable 'key' is is not defined. I think I've defined it here
You only wrote key.ch[0]

key.ch[1] was untouched, and key.i is basically junk (partially initialised by it's overlap with key[0]).

Thanks Salem for your very much helping reply. Now I get it what I was wrong with. Thanks.