Endianness (again)

[gamehack]

Hello all,

Sorry for asking too much but I've been playing with some code and just
wrote this function:

void reverse(uint32_t num)
{
uint8_t bytes[4];
printf("sizeof(uint8_t) is %d and bits per byte is: %d\n",
sizeof(uint8_t), CHAR_BIT);
bytes[0] = (uint8_t) ((num & 0x000000FF) >> 0);
bytes[1] = (uint8_t) ((num & 0x0000FF00) >> 8);
bytes[2] = (uint8_t) ((num & 0x00FF0000) >> 16);
bytes[3] = (uint8_t) ((num & 0xFF000000) >> 24);

printf("Number in memory: %#x %#x %#x %#x\nActual num is: %#x\n",
bytes[3], bytes[2], bytes[1], bytes[0], num);
}

And on my little endian machine it outputs:

sizeof(uint8_t) is 1 and bits per byte is: 8
Number in memory: 0xaa 0xbb 0xcc 0xdd
Actual num is: 0xaabbccdd

Surely the number in memory has to be 0xdd 0xcc 0xbb 0xaa... I'm doing
some really stupid mistake but I can't see it.

Thanks

 

[Lucien Kennedy-Lamb]

Hello all,

Sorry for asking too much but I've been playing with some code and just
wrote this function:

void reverse(uint32_t num)
{
uint8_t bytes[4];
printf("sizeof(uint8_t) is %d and bits per byte is: %d\n",
sizeof(uint8_t), CHAR_BIT);
bytes[0] = (uint8_t) ((num & 0x000000FF) >> 0);
bytes[1] = (uint8_t) ((num & 0x0000FF00) >> 8);
bytes[2] = (uint8_t) ((num & 0x00FF0000) >> 16);
bytes[3] = (uint8_t) ((num & 0xFF000000) >> 24);

printf("Number in memory: %#x %#x %#x %#x\nActual num is: %#x\n",
bytes[3], bytes[2], bytes[1], bytes[0], num);
}

And on my little endian machine it outputs:

sizeof(uint8_t) is 1 and bits per byte is: 8
Number in memory: 0xaa 0xbb 0xcc 0xdd
Actual num is: 0xaabbccdd

Surely the number in memory has to be 0xdd 0xcc 0xbb 0xaa... I'm doing
some really stupid mistake but I can't see it.

Thanks

There is nothing in your code which relies on endianess!

You are explicity storing 8-bit chunks of the value in num to bytes and
then printing them out (in reverse order which accounts for your
confusion).

If you looked at the memory at &num then you would see 0xdd 0xcc 0xbb
0xaa.


uint8_t *bytes = (uint8_t*)#

printf("Number in memory: %#x %#x %#x %#x\nActual num is: %#x\n",
bytes[0], bytes[1], bytes[2], bytes[3], num);

Lucien Kennedy-Lamb

 

[Rod Pemberton]

Hello all,

Sorry for asking too much but I've been playing with some code and just
wrote this function:

void reverse(uint32_t num)
{
uint8_t bytes[4];
printf("sizeof(uint8_t) is %d and bits per byte is: %d\n",
sizeof(uint8_t), CHAR_BIT);
bytes[0] = (uint8_t) ((num & 0x000000FF) >> 0);
bytes[1] = (uint8_t) ((num & 0x0000FF00) >> 8);
bytes[2] = (uint8_t) ((num & 0x00FF0000) >> 16);
bytes[3] = (uint8_t) ((num & 0xFF000000) >> 24);

printf("Number in memory: %#x %#x %#x %#x\nActual num is: %#x\n",
bytes[3], bytes[2], bytes[1], bytes[0], num);
}

And on my little endian machine it outputs:

sizeof(uint8_t) is 1 and bits per byte is: 8
Number in memory: 0xaa 0xbb 0xcc 0xdd
Actual num is: 0xaabbccdd

Surely the number in memory has to be 0xdd 0xcc 0xbb 0xaa... I'm doing
some really stupid mistake but I can't see it.

Thanks

The second case in my reply to Abhishek "How to determine the data is stored
in memory" 1/22/06 12:52 pm, might help you solve your problem..

Rod Pemberton

 

[gamehack]

Yeah! I got it I just didn't get the address of the actual bytes.

Thanks

Hello all,

[snip]

The second case in my reply to Abhishek "How to determine the data is stored
in memory" 1/22/06 12:52 pm, might help you solve your problem..

Rod Pemberton

 

[Nelu]

Yeah! I got it I just didn't get the address of the actual bytes.

Thanks

Please don't top-post.

 

[CBFalconer]

Sorry for asking too much but I've been playing with some code and
just wrote this function:

void reverse(uint32_t num)
{
uint8_t bytes[4];
printf("sizeof(uint8_t) is %d and bits per byte is: %d\n",
sizeof(uint8_t), CHAR_BIT);
bytes[0] = (uint8_t) ((num & 0x000000FF) >> 0);
bytes[1] = (uint8_t) ((num & 0x0000FF00) >> 8);
bytes[2] = (uint8_t) ((num & 0x00FF0000) >> 16);
bytes[3] = (uint8_t) ((num & 0xFF000000) >> 24);

printf("Number in memory: %#x %#x %#x %#x\nActual num is: %#x\n",
bytes[3], bytes[2], bytes[1], bytes[0], num);
}

And on my little endian machine it outputs:

sizeof(uint8_t) is 1 and bits per byte is: 8
Number in memory: 0xaa 0xbb 0xcc 0xdd
Actual num is: 0xaabbccdd

Surely the number in memory has to be 0xdd 0xcc 0xbb 0xaa... I'm doing
some really stupid mistake but I can't see it.

Try:
printf("Number in memory: %#x %#x %#x %#x\nActual num is:
%#x\n",
bytes[0], bytes[1], bytes[2], bytes[3], num);

Better yet, use:

printf("Number in memory:");
for (i = 0; i < 4; i++) printf(" %x", bytes);
printf("\nActual num is: %#x\n", num);

Long rambling statements are not conducive to accuracy.

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"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell.org/google/>

 

[=?ISO-8859-1?Q?=22Nils_O=2E_Sel=E5sdal=22?=]

Hello all,

Sorry for asking too much but I've been playing with some code and just
wrote this function:

void reverse(uint32_t num)
{
uint8_t bytes[4];
printf("sizeof(uint8_t) is %d and bits per byte is: %d\n",
sizeof(uint8_t), CHAR_BIT);
bytes[0] = (uint8_t) ((num & 0x000000FF) >> 0);
bytes[1] = (uint8_t) ((num & 0x0000FF00) >> 8);
bytes[2] = (uint8_t) ((num & 0x00FF0000) >> 16);
bytes[3] = (uint8_t) ((num & 0xFF000000) >> 24);

printf("Number in memory: %#x %#x %#x %#x\nActual num is: %#x\n",
bytes[3], bytes[2], bytes[1], bytes[0], num);
}

And on my little endian machine it outputs:

sizeof(uint8_t) is 1 and bits per byte is: 8
Number in memory: 0xaa 0xbb 0xcc 0xdd
Actual num is: 0xaabbccdd

Surely the number in memory has to be 0xdd 0xcc 0xbb 0xaa... I'm doing
some really stupid mistake but I can't see it.

You are writing a uint32 to a sequence of uint8_t in a portable(to the
implementations that provides these types) way in big endian
representation. It will output the same thing on a big endian
machine.

Great !