Using 1 int to send 2 numbers

Question

I am working with a system whereby I can send 1 32 bit int at a time. However I need to send two numbers at a time. What is the best way to do this in standard C?

I assume I will have to do some conversion to binary/hex and some masking?

Any ideas would be appreciated.

Thanks

What is the range of each number? How many bits do you need to send each number? — Joe Cullity
– Joe Cullity, Commented Mar 13, 2011 at 18:02
Do you mean "send over a network" or "send from one part of my program to another"? What's the range of possible values of the numbers you want to pack into a single 32-bit integer? — Gareth McCaughan
– Gareth McCaughan, Commented Mar 13, 2011 at 18:02
@delnan, I think David H. meant a union containing an int and a struct. — Gareth McCaughan
– Gareth McCaughan, Commented Mar 13, 2011 at 18:03

Thomas · Accepted Answer · 2011-03-13 19:01:37Z

4

You can encode two 16-bit unsigned numbers like this:

unsigned int out = (in1 << 16) | in2;

and decode them like this:

unsigned int in1 = out >> 16;
unsigned int in2 = out & 0xFFFF;

All this assumes that int is at least 32 bits, that in1 and in2 are in the 0-65535 range, and that an unsigned int can be sent across correctly (w.r.t. endianness).

edited Mar 13, 2011 at 19:01

answered Mar 13, 2011 at 18:02

Thomas

183k57 gold badges383 silver badges510 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Mat Over a year ago

That works as long as the sender and receiver have the same endianess. If that's going "on the wire", care must be taken (i.e. see the htons family of functions).

xanatos Over a year ago

+1 But the +1 should go to Mat, because his caveat is more important than your solution (that is good too)... And perhaps you could explain that with "signed" ints all bets are off (because shift is undefined with integer < 0 )

Ferguzz Over a year ago

The only data types I have at my disposal are int (32) and float (32) but i'll have a go now at making something like this work. Thanks!

razlebe Over a year ago

endianess - a word I'm going to try and get into as many conversations as possible this week, for sport. :)

Steve Jessop Over a year ago

+1, if "the system" that already exists can sent a 32 bit int correctly, then it already accounts for endian-ness, and therefore this code will work regardless of the endian-ness of the sender or the recipient. If the existing system doesn't deal with endian-ness correctly, then that's a problem, but not with this answer. I suppose there's a slight niggle that this code doesn't work if the channel only does signed int, and the sending C implementation can't convert large unsigned values to signed. You'd create your unsigned int for sending, and now what? But that's rather rare.

|

pmg · Accepted Answer · 2011-03-13 18:03:44Z

2

If the numbers you are sending are between 0 and 65535 each just pack them in the 32-bit int you can send:

unsigned int n1 = 42;
unsigned int n2 = 1600;
unsigned int numbertosend = (n1 << 16) | n2;

On the receiving side, unpack the number

unsigned int n1 = receivednumber >> 16;
unsigned int n2 = receivednumber & 0xFFFF;

answered Mar 13, 2011 at 18:03

pmg

109k14 gold badges132 silver badges203 bronze badges

Comments

Keith · Accepted Answer · 2011-03-13 18:04:33Z

1

What range are your numbers? If they can fit in 16 bits, then you can pack two of those in your 32 bit int. Something like i = (n1 << 16) | (n2 &0xffff).

answered Mar 13, 2011 at 18:04

Keith

43.2k11 gold badges62 silver badges77 bronze badges

Comments

Theon Lin · Accepted Answer · 2011-03-13 18:08:44Z

1

You could encode 2 16-bit numbers into a 32-bit number. For example:

int32 encode(int16 numA, int16 numb) {
    int 32 result = numA << 16 | numB;
    return  result;
}

int16 decodeNum1(int32 num) {
    return (num >> 16) & 0xFFFF;
}

int16 decodeNum2(int32 num) {
    return (num) & 0xFFFF;
}

answered Mar 13, 2011 at 18:08

Theon Lin

3303 silver badges8 bronze badges

Comments

necromancer · Accepted Answer · 2011-03-13 22:06:44Z

1

If the two numbers you want to send are both prime, you could multiply them and send the product, and on the server factor them to get the result.

;p (inspired by the ambiguity in the question)

answered Mar 13, 2011 at 22:06

necromancer

24.7k22 gold badges72 silver badges118 bronze badges

Comments

E.Benoît · Accepted Answer · 2011-03-13 18:02:24Z

0

Assuming you want to send 2 16-bit numbers, you could probably do something like this:

unsigned int a, b, toSend;
// Give a and b a value...
toSend = ( a << 16 ) | b;

answered Mar 13, 2011 at 18:02

E.Benoît

8067 silver badges10 bronze badges

Comments

Algorithmist · Accepted Answer · 2011-03-13 18:03:15Z

0

I think Unions in c would do the job.You could also go for Bit Fields in C

answered Mar 13, 2011 at 18:03

Algorithmist

6,7237 gold badges38 silver badges50 bronze badges

Collectives™ on Stack Overflow

Using 1 int to send 2 numbers

7 Answers 7

8 Comments

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

7 Answers 7

8 Comments

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related