integer encoding & byte ordering

slzckboy · 07-07-2005, 10:42 AM

ok... it was my rusty maths that was giving me the wrong output.
I was calculating my remainder wrong.
The algorithm that jlliagre gave me initially does indeed give me the desired result.

Can someone explain why an integer would be encoded in this manner please ?

keefaz · 07-07-2005, 02:22 PM

Which manner ? 32 bits or byte ordering ?

slzckboy · 07-07-2005, 02:41 PM

I understand the need for a standard network byte odering;and that diffent computer architectures use different byte ordering etc,
I feel I am missing the point with something basic however.
Using my first example.Why when transmitting the value 17,998,720 byte ordering aside , is this number not just transmitted as 17 99 87 20 ?
why re-encode to 1 18 163 128 ?

cheers.

jlliagre · 07-07-2005, 03:00 PM

Computers are storing information in binary (0 and 1), when a number is stored or transmitted, bits, not digits are involved. To represent a 32 bit integer, it is easier to show it using bytes (8 bits numbers) which span from 0 to 255, the numbers 1 18 163 128 represent the four bytes that when combined make 17998720.
This is far more efficient than storing them in decimal, like you suggest (60% better per digit actually).

slzckboy · 07-07-2005, 03:07 PM

And this needs to be implemented by the programmer,rather then beng carried out automatically by the socket API then?

The book I'm reading isn't clear on this point.

exvor · 07-08-2005, 02:22 PM

Ahh yes. ya know the strange thing is that i dident get an compiler error or warning ??

maybe i did it right in the real code ill have to double check.

aluser · 07-08-2005, 02:46 PM

Quote:

And this needs to be implemented by the programmer,rather then beng carried out automatically by the socket API then?

Which thing are you asking about? There are some convenience functions ntohl, htonl, ntohs, and htons to convert to and from big endian order which is standard for sending between hosts. Other than that, there's no reason that you'd have to read each byte of your integer that's related particularly to sockets...

That integers are stored as sequences of bits and operated on with twos complement arithmetic is a fundamental property of your processor. You can transmit numbers as ASCII if you want, and it'll be slow and (imo) harder to parse. Easier to look at in a packet capturer though. See atoi(), strtol(), and sprintf() for converting to and from ASCII.

I guess that's all a bit much detail. An important point is that you can just dump the internal representation of an integer out a socket like this:

Code:

int32_t a = 2354234;
byteswritten = send(sock, &a, sizeof(a), 0);
/* hopefully byteswritten == sizeof(a) == 4 */

and read it on the other end like

Code:

int32_t a;
bytesread = recv(sock, &a, sizeof(a), 0);
/* hopefully bytesread == sizeof(a) == 4 */

If one end of the connection is a PC (little endian) and the other is a mac (big endian) then a will come over as a different number. oops. A solution is to always transmit in network (big endian) order. So really the sender always should do

Code:

a = htonl(a);

before sending and the receiver should always do

Code:

a = ntohl(a);

after receiving. Now the endianness of the two hosts' processors doesn't matter. That's how you transmit numbers over a network without converting to ascii. No fancy math necessary.

slzckboy · 07-08-2005, 04:38 PM

I understand.
Thnks 4 your patients.

exvor · 07-08-2005, 07:29 PM

yea i was about to post something similar that there are functions to convert
your data or ascii into ither format.

Im reading something similar on C and sockets

fun stuff too bad i dont understand some of the more complicated struct stuff here.

slzckboy · 07-09-2005, 03:52 PM

..Yeah it is fun ...especially wen u have a "eureka" momment and understand something that u were stuck on.
I think i could implement to algorithum in C now if I needed to but I have no idea how the thing was derived.I guess as there is a suite of programs that do the conversion for me I shall not loose sleep for now.

i'm a programming noob and am in the middle of developing a simple irc client,(re-inventing the wheel I know) as an educational excercise in C/Linux.

Thnks again to those who have contributed to making this an enlightening thread.

eddiebaby1023 · 07-09-2005, 04:40 PM

Although converting to network order is The Right Thing to do, another way to hack an int is (assuming 32-bit ints):

Code:

union {
    int i;
    char[4];
}

Load the int and read the chars in the desired order, 0-3 or 3-0, as the mood takes you.