Represenation of char type

[sarojini]

Hi,

I have below piece of code :

#include <stdio.h>

void main()
{
union a
{
	int i;
	char ch[2];
};
union a key;
key.i=512;
printf("\n key.i= %d",key.i);
printf("\n key.ch[0]= %d",key.ch[0]);
printf("\n key.ch[1]= %d",key.ch[1]);

key.ch[0]=50;
printf("\n key.i= %d",key.i);
printf("\n key.ch[0]= %d",key.ch[0]);
printf("\n key.ch[1]= %d",key.ch[1]); 

}

Results:
-----------
For the first set when key.i is 512,
key.ch[0]=0 and key.ch[1]=2 and key.i=512

However for second set of data ie when ch[0]=50 its showing as
key.i=562
key.ch[0]=50 and key.ch[1]=2

Can anyone please explain me how key.i is 562? This stuff is something to do with endianness.

[jeffcobb]

Because in your first int the binary representation is 1000000000 (leading zeros deleted; I am a lazy git). By setting the low-order bits to 50 (110010) you end up with [11] [00110010] = 562. Or to put it another way for the mathematically challenged, since after the first setting of the int the low-order byte is zero, your math becomes essentially 512 + 50 = 562. Did you expect something different? BTW nothing to do with endian-ness..

[sarojini]

Hi Jeff,

I get 0000000000000010 in memory representation when key.i=512

Now when key.ch[0]=50
the low order bits will be 0110010
Since its one byte data it gets overwitten and it will be finally 0000000000110010

You said "By setting the low-order bits to 50 (110010) you end up with [11] [00110010] = 562."

Could you please me how did you get [11] [00110010]

Thank you

[jeffcobb]

Because you have your bytes out of order sir. Look more closely at my representation of the number 512: 1000000000. Thus in this case the first byte (reading right-to-left) is 00000000 or zero and the second byte (reading the same way) is 10 (leading zeros deleted). Setting byte zero as you have done simple changes or adds the number 50 to your initial value.

Oh and as for the [11] that was a mistake as I was doing the binary off of the top of my head; it should be [10] [00110010]

Sorry. What I get for doing binary in my head before coffee.

[sarojini]

Thank you Jeff

[Elysia]

Btw, you should int main, not void main.