Question

int i=512;
char *c = (char *)&i;
c[0] =1;
printf("%d",i);

this displays “513”, it adds 1 to i.

int i=512;
char *c = (char *)&i;
c[1] =1;
printf("%d",i);

whereas this displays 256. Divides it by 2.
Can someone please explain why? thanks a lot

Answer 1

Binary

The 32-bit number 512 expressed in binary, is just:

00000000000000000000001000000000

because 2 to the power of 9 is 512. Conventionally, you read the bits from right-to-left.

Here are some other decimal numbers in binary:

0001 = 1
0010 = 2
0011 = 3
0100 = 4

The Cast: Reinterpreting the Int as an Array of Bytes

When you do this:

int i = 512;
char *c = (char *)&i;

you are interpreting the 4-byte integer as an array of characters (8-bit bytes), as you probably know. If not, here’s what’s going on:

&i

takes the address of the variable i.

(char *)&i

reinterprets it (or casts it) to a pointer to char type. This means it can now be used like an array. Since you know an int is at least 32-bit on your machine, can access its bytes using c[0], c[1], c[2], c[3].

Depending on the endianness of the system, the bytes of the number might be laid out: most significant byte first (big endian), or least significant byte first (little endian). x86 processors are little endian. This basically means the number 512 is laid out as in the example above, i.e.:

00000000 00000000 00000010 00000000
    c[3]     c[2]     c[1]     c[0]

I’ve grouped the bits into separate 8-bit chunks (bytes) corresponding to the way they are laid out in memory. Note, you also read them right-to-left here, so we can keep with conventions for the binary number system.

Consequences

Now setting c[0] = 1 has this effect:

00000000 00000000 00000010 00000001
    c[3]     c[2]     c[1]     c[0]

which is 2^9 + 2^0 == 513 in decimal.

Setting c[1] = 1 has this effect:

00000000 00000000 00000001 00000000
    c[3]     c[2]     c[1]     c[0]

which is 2^8 == 256 in decimal, because you’ve overwritten the second byte 00000010 with 00000001

Do note on a big endian system, the bytes would be stored in reverse order to a little endian system. This would mean you’d get totally different results to ones you got if you ran it on one of those machines.