# swapping bytes in an integer

Discussion in 'C Programming' started by john@fcs.uga.edu, May 8, 2006.

1. ### Guest

If I have a 32 bit unsigned int that is in the wrong byte order, how
can I convert it? For example, if have a number 0x090a0b0c how can I
reverse this to 0x0c0b0a09 ?

Thanks,
-John

, May 8, 2006

2. ### W MarshGuest

On 8 May 2006 14:03:08 -0700, wrote:

>If I have a 32 bit unsigned int that is in the wrong byte order, how
>can I convert it? For example, if have a number 0x090a0b0c how can I
>reverse this to 0x0c0b0a09 ?
>
>Thanks,
>-John

The shift operators << and >> shift a value left and right by one bit,
respectively. Shifting by 4 bits will move the value by one hex digit.
You can combine values with the bitwise OR operator, |

W Marsh, May 8, 2006

3. ### Flash GordonGuest

wrote:
> If I have a 32 bit unsigned int that is in the wrong byte order, how
> can I convert it? For example, if have a number 0x090a0b0c how can I
> reverse this to 0x0c0b0a09 ?

Depends on whether you want to convert it in place or not. Anyway, the
simple way to understand is using shift operators >> and << and bitwise
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

Flash Gordon, May 8, 2006
4. ### Ian CollinsGuest

wrote:
> If I have a 32 bit unsigned int that is in the wrong byte order, how
> can I convert it? For example, if have a number 0x090a0b0c how can I
> reverse this to 0x0c0b0a09 ?
>

It my be non-standard, bit if you system has the networking library
function ntohl and you don't require portability, you can use that.

--
Ian Collins.

Ian Collins, May 8, 2006
5. ### TomásGuest

> Monkey monkey = 0x090a0b0c;

Opps! Should have written:

Monkey monkey;

monkey.four_bytes = 0x090a0b0c;

-Tomás

Tomás, May 8, 2006
6. ### W MarshGuest

On Mon, 08 May 2006 21:35:03 GMT, "Tomás" <> wrote:

>
>> Monkey monkey = 0x090a0b0c;

>
>
>Opps! Should have written:
>
>
> Monkey monkey;
>
> monkey.four_bytes = 0x090a0b0c;
>
>
>-Tomás

It makes no difference - we have no idea what Monkey is anyway. It was
probably simpler the first way, we could just assume it was an integer
of some description.

W Marsh, May 8, 2006
7. ### Keith ThompsonGuest

W Marsh <> writes:
> On 8 May 2006 14:03:08 -0700, wrote:
>>If I have a 32 bit unsigned int that is in the wrong byte order, how
>>can I convert it? For example, if have a number 0x090a0b0c how can I
>>reverse this to 0x0c0b0a09 ?

>
> The shift operators << and >> shift a value left and right by one bit,
> respectively. Shifting by 4 bits will move the value by one hex digit.
> You can combine values with the bitwise OR operator, |

No, the shift operators shift a value by a specified number of bits.
For example, x<<1 yields x shifted left by one bit, x<<4 yields x
shifted left by 4 bits, and x<<N yields x shifted left by N bits.

--
Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Keith Thompson, May 8, 2006
8. ### =?utf-8?B?SGFyYWxkIHZhbiBExLNr?=Guest

Ian Collins wrote:
> wrote:
> > If I have a 32 bit unsigned int that is in the wrong byte order, how
> > can I convert it? For example, if have a number 0x090a0b0c how can I
> > reverse this to 0x0c0b0a09 ?
> >

> It my be non-standard, bit if you system has the networking library
> function ntohl and you don't require portability, you can use that.

If I understand correctly, ntohl() converts a big endian integer to a
system integer, meaning if system integers are already big endian, it
simply returns whatever you pass it. So it may not be useful even when
it does exist.

Treating the number when it has the wrong byte order as an array of
unsigned char, and manually combining octets (or manually splitting the
number into them, if you have to go the other way), is a more portable
alternative, which if done right should work on any system.

=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=, May 8, 2006
9. ### W MarshGuest

On Mon, 08 May 2006 21:53:59 GMT, Keith Thompson <>
wrote:

>W Marsh <> writes:
>> On 8 May 2006 14:03:08 -0700, wrote:
>>>If I have a 32 bit unsigned int that is in the wrong byte order, how
>>>can I convert it? For example, if have a number 0x090a0b0c how can I
>>>reverse this to 0x0c0b0a09 ?

>>
>> The shift operators << and >> shift a value left and right by one bit,
>> respectively. Shifting by 4 bits will move the value by one hex digit.
>> You can combine values with the bitwise OR operator, |

>
>No, the shift operators shift a value by a specified number of bits.
>For example, x<<1 yields x shifted left by one bit, x<<4 yields x
>shifted left by 4 bits, and x<<N yields x shifted left by N bits.

I meant to write "shift a value left and right through bits", yes.

W Marsh, May 8, 2006
10. ### Ian CollinsGuest

Harald van DÄ³k wrote:
> Ian Collins wrote:
>
>> wrote:
>>
>>>If I have a 32 bit unsigned int that is in the wrong byte order, how
>>>can I convert it? For example, if have a number 0x090a0b0c how can I
>>>reverse this to 0x0c0b0a09 ?
>>>

>>
>>It my be non-standard, bit if you system has the networking library
>>function ntohl and you don't require portability, you can use that.

>
>
> If I understand correctly, ntohl() converts a big endian integer to a
> system integer, meaning if system integers are already big endian, it
> simply returns whatever you pass it. So it may not be useful even when
> it does exist.
>

Yes, you are correct. My advice was wrong.

--
Ian Collins.

Ian Collins, May 8, 2006
11. ### Guest

Ian Collins wrote:
> wrote:
> > If I have a 32 bit unsigned int that is in the wrong byte order, how
> > can I convert it? For example, if have a number 0x090a0b0c how can I
> > reverse this to 0x0c0b0a09 ?
> >

> It my be non-standard, bit if you system has the networking library
> function ntohl and you don't require portability, you can use that.
>
> --
> Ian Collins.

I just tried ntohl() in my program and this works perfectly. I think I
will use this method. I want to thank everyone who replied for all of
their great ideas! This help is greatly appreciated.

-John

, May 8, 2006
12. ### W MarshGuest

On 8 May 2006 15:18:42 -0700, wrote:

>Ian Collins wrote:
>> wrote:
>> > If I have a 32 bit unsigned int that is in the wrong byte order, how
>> > can I convert it? For example, if have a number 0x090a0b0c how can I
>> > reverse this to 0x0c0b0a09 ?
>> >

>> It my be non-standard, bit if you system has the networking library
>> function ntohl and you don't require portability, you can use that.
>>
>> --
>> Ian Collins.

>
>I just tried ntohl() in my program and this works perfectly. I think I
>will use this method. I want to thank everyone who replied for all of
>their great ideas! This help is greatly appreciated.
>
>-John

Then bear in mind that it's not portable, and will have no effect when
compiled for a machine with a different endianess to your own.

W Marsh, May 8, 2006
13. ### Andrew PoelstraGuest

On 2006-05-08, W Marsh <> wrote:
> On 8 May 2006 15:18:42 -0700, wrote:
>
>>Ian Collins wrote:
>>> wrote:
>>> > If I have a 32 bit unsigned int that is in the wrong byte order, how
>>> > can I convert it? For example, if have a number 0x090a0b0c how can I
>>> > reverse this to 0x0c0b0a09 ?
>>> >
>>> It my be non-standard, bit if you system has the networking library
>>> function ntohl and you don't require portability, you can use that.
>>>
>>> --
>>> Ian Collins.

>>
>>I just tried ntohl() in my program and this works perfectly. I think I
>>will use this method. I want to thank everyone who replied for all of
>>their great ideas! This help is greatly appreciated.
>>
>>-John

>
> Then bear in mind that it's not portable, and will have no effect when
> compiled for a machine with a different endianess to your own.

If his code depends on big-endianness, then it doesn't matter that the function
has no effect; it simply means that the machine is already set up correctly

I worded that really poorly, but basically I mean that it is irrelevant what
effect a certain function actually has; only the end result is important.

Andrew Poelstra, May 8, 2006
14. ### W MarshGuest

On Mon, 08 May 2006 22:41:34 GMT, Andrew Poelstra
<> wrote:

>On 2006-05-08, W Marsh <> wrote:
>> On 8 May 2006 15:18:42 -0700, wrote:
>>
>>>Ian Collins wrote:
>>>> wrote:
>>>> > If I have a 32 bit unsigned int that is in the wrong byte order, how
>>>> > can I convert it? For example, if have a number 0x090a0b0c how can I
>>>> > reverse this to 0x0c0b0a09 ?
>>>> >
>>>> It my be non-standard, bit if you system has the networking library
>>>> function ntohl and you don't require portability, you can use that.
>>>>
>>>> --
>>>> Ian Collins.
>>>
>>>I just tried ntohl() in my program and this works perfectly. I think I
>>>will use this method. I want to thank everyone who replied for all of
>>>their great ideas! This help is greatly appreciated.
>>>
>>>-John

>>
>> Then bear in mind that it's not portable, and will have no effect when
>> compiled for a machine with a different endianess to your own.

>
>If his code depends on big-endianness, then it doesn't matter that the function
>has no effect; it simply means that the machine is already set up correctly
>
>I worded that really poorly, but basically I mean that it is irrelevant what
>effect a certain function actually has; only the end result is important.

No - he asked how to swap bytes, not how to manifest a specific
endianess.

If he has data he wants to treat natively, then ntohl is fine.

If he actually wants to swap the bytes of an integer in any case
(implementing a swap opcode in a CPU emulator, for example), he won't
be able to use ntohl.

W Marsh, May 8, 2006
15. ### EventHelix.comGuest

EventHelix.com, May 9, 2006
16. ### Guest

EventHelix.com wrote:
> > If I have a 32 bit unsigned int that is in the wrong byte order, how
> > can I convert it? For example, if have a number 0x090a0b0c how can I
> > reverse this to 0x0c0b0a09 ?

>
> The following article should help:
> http://www.eventhelix.com/RealtimeMantra/ByteAlignmentAndOrdering.htm
>
> --
> EventStudio System Designer 2.5 - http://www.EventHelix.com/EventStudio
> Sequence Diagram Based System Design and Object Interaction Modeling
> Tool

>From this web site...

Routines to convert between big-endian and little-endian formats are
actually quite straight forward. The routines shown below will convert
from both ways, i.e. big-endian to little-endian and back.
Big-endian to Little-endian conversion and back

short convert_short(short in)
{
short out;
char *p_in = (char *) &in;
char *p_out = (char *) &out;
p_out[0] = p_in[1];
p_out[1] = p_in[0];
return out;
}

long convert_long(long in)
{
long out;
char *p_in = (char *) &in;
char *p_out = (char *) &out;
p_out[0] = p_in[3];
p_out[1] = p_in[2];
p_out[2] = p_in[1];
p_out[3] = p_in[0];
return out;
}

, May 9, 2006
17. ### Rod PembertonGuest

<> wrote in message
news:...
> If I have a 32 bit unsigned int that is in the wrong byte order, how
> can I convert it? For example, if have a number 0x090a0b0c how can I
> reverse this to 0x0c0b0a09 ?
>

It seems you've found ntohl(), but this is how:

unsigned long a,b;

a=0x090a0b0c;
b=((a&0xFF000000)>>24)|((a&0x00FF0000)>>8)|((a&0x0000FF00)<<8)|((a&0x000000F
F)<<24);

Of course, you can drop any place holder zeros between 0x and FF.

Rod Pemberton

Rod Pemberton, May 9, 2006
18. ### Eric SosmanGuest

wrote On 05/08/06 18:18,:
> Ian Collins wrote:
>
>> wrote:
>>
>>>If I have a 32 bit unsigned int that is in the wrong byte order, how
>>>can I convert it? For example, if have a number 0x090a0b0c how can I
>>>reverse this to 0x0c0b0a09 ?
>>>

>>
>>It my be non-standard, bit if you system has the networking library
>>function ntohl and you don't require portability, you can use that.
>>
>>--
>>Ian Collins.

>
>
> I just tried ntohl() in my program and this works perfectly. I think I
> will use this method. I want to thank everyone who replied for all of
> their great ideas! This help is greatly appreciated.

<off-topic reason="potential bug">

Be careful: ntohl() does *not* do what you asked.
It converts "network byte order" (Big-Endian) to "host
byte order" (whatever your machine uses). If your
machine uses Big-Endian byte order already, ntohl()
will not swap the bytes: it will do nothing at all.
To put it another way, ntohl() may in fact do what you
want on the machine you're using at the moment, but
will definitely *not* do what you want on all machines.

If you need to swap the bytes unconditionally, no
matter what machine you're using, you'll have to work
a little harder.

</off-topic>

--

Eric Sosman, May 9, 2006
19. ### Guest

Wow! This code is really awesome. I think this is the absolute best
way I have seen yet. I will have to study your code and break it down
to see how it works exaclty. Do you think that it could it be placed
within a #define ?

Thanks,
-John

, May 9, 2006
20. ### Keith ThompsonGuest

Eric Sosman <> writes:
> wrote On 05/08/06 18:18,:
>> Ian Collins wrote:
>>> wrote:
>>>>If I have a 32 bit unsigned int that is in the wrong byte order, how
>>>>can I convert it? For example, if have a number 0x090a0b0c how can I
>>>>reverse this to 0x0c0b0a09 ?
>>>
>>>It my be non-standard, bit if you system has the networking library
>>>function ntohl and you don't require portability, you can use that.

>>
>> I just tried ntohl() in my program and this works perfectly. I think I
>> will use this method. I want to thank everyone who replied for all of
>> their great ideas! This help is greatly appreciated.

>
> <off-topic reason="potential bug">
>
> Be careful: ntohl() does *not* do what you asked.
> It converts "network byte order" (Big-Endian) to "host
> byte order" (whatever your machine uses). If your
> machine uses Big-Endian byte order already, ntohl()
> will not swap the bytes: it will do nothing at all.
> To put it another way, ntohl() may in fact do what you
> want on the machine you're using at the moment, but
> will definitely *not* do what you want on all machines.
>
> If you need to swap the bytes unconditionally, no
> matter what machine you're using, you'll have to work
> a little harder.

We're not certain exactly what the OP's requirements are, only what he
stated them to be. He might need a general solution to swapping bytes
(in which case ntohl() won't do it), or it may be that the only time
he'll see a number in the wrong byte order is when it comes from a
network (in which case ntohl() is probably just the thing).

> </off-topic>

--
Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Keith Thompson, May 9, 2006