converting a big endian to little endian

[junky_fellow]

Hi guys,

I need to convert a big endian integer to little endian integer.
(the integer is 4 bytes in size on my implementation) . I came up with
the following code. I need your comments on this. Please suggest any
improvements that can be done.

#include <stdio.h>
int main(void)
{
int big = 0x12345678;
int little;
char *big_ptr = (char *)&big;
char *little_ptr = (char *)&little;

little_ptr[0] = big_ptr[3];
little_ptr[1] = big_ptr[2];
little_ptr[2] = big_ptr[1];
little_ptr[3] = big_ptr[0];

printf("big = 0x%x little = 0x%x\n",big,lit tle);

}

[loic-dev]

Hello,

I need to convert a big endian integer to little endian integer.
(the integer is 4 bytes in size on my implementation) . I came up with
the following code. I need your comments on this. Please suggest any
improvements that can be done.

You can use 'shift' combined with 'and' operator to get the same
result:

/*
* convert big -little endian and conversly
* this function assumes that sizeof(unsigned int) == 4
*/
unsigned int
endian_swap(uns igned int x)
{
return
(x>>24) |
((x>>8) & 0x0000ff00) |
((x<<8) & 0x00ff0000) |
(x<<24);
}

Cheers,
Loic.

[David T. Ashley]

<ju**********@y ahoo.co.inwrote in message
news:11******** **************@ 80g2000cwy.goog legroups.com...

Hi guys,

I need to convert a big endian integer to little endian integer.
(the integer is 4 bytes in size on my implementation) . I came up with
the following code. I need your comments on this. Please suggest any
improvements that can be done.

#include <stdio.h>
int main(void)
{
int big = 0x12345678;
int little;
char *big_ptr = (char *)&big;
char *little_ptr = (char *)&little;

little_ptr[0] = big_ptr[3];
little_ptr[1] = big_ptr[2];
little_ptr[2] = big_ptr[1];
little_ptr[3] = big_ptr[0];

printf("big = 0x%x little = 0x%x\n",big,lit tle);

}

The code above should work fine. However, the more traditional approach is
to use a union, i.e.

union
{
int i;
char c[4];
} pickapart;

pickapart.i = input_arg;
output0 = pickapart.c[0];

However, if you can find an approach involving shifting only, and if an
examination of the assembly-language shows that the compiler does it well
(probably without shifting), that would be the preferred approach, i.e.

output0 = input_arg & 0xFF;
output1 = (input_arg >>8) & 0xFF;
etc.

then that would be the preferred approach. The approach you cited can lead
to memory addressing problems if the int size is <32, and the approach I
cited with the union won't work for other sizes of integers. There should
be a more portable and general approach.

[Anoop Saxena]

Hi,
New here, so apologies if I have not followed any rules while
adding code snippets etc. Do let me know.

endian_swap(uns igned int x)
{
return
(x>>24) |
((x>>8) & 0x0000ff00) |
((x<<8) & 0x00ff0000) |
(x<<24);
}

Cheers,
Loic.

If int is 32 bits, then how good is the following code for unsigned
int's?

int main(void)
{
unsigned int x = 0xffaa2211;
unsigned int z = 0;
z = ( (x << 16) | ( x >16) );

return 0;
}

[Chris Dollin]

Anoop Saxena wrote:

If int is 32 bits, then how good is the following code for unsigned
int's?

int main(void)
{
unsigned int x = 0xffaa2211;
unsigned int z = 0;
z = ( (x << 16) | ( x >16) );

return 0;
}

Broken. It will generate 0x2211ffaa, not the required
0x1122aaff.

(It also has /definitely/ superfluous brackets and an
unnecessary initialisation of `z`, which are style issues,
and it would have been nice for there to be some kind
of output.)

--
Chris "Perikles triumphant" Dollin
Scoring, bah. If I want scoring I'll go play /Age of Steam/.

[Random832]

2006-12-14 <el**********@m urdoch.hpl.hp.c om>,
Chris Dollin wrote:

Anoop Saxena wrote:

> z = ( (x << 16) | ( x >16) );

(It also has /definitely/ superfluous brackets

Precedence rules for bitwise operators are not widely understood.

[Chris Dollin]

Random832 wrote:

2006-12-14 <el**********@m urdoch.hpl.hp.c om>,
Chris Dollin wrote:

>Anoop Saxena wrote:

>> z = ( (x << 16) | ( x >16) );

(It also has /definitely/ superfluous brackets

Precedence rules for bitwise operators are not widely understood.

I don't doubt it, but those aren't the ones I was talking about.

--
Chris "Perikles triumphant" Dollin
"Our future looks secure, but it's all out of our hands"
- Magenta, /Man and Machine/

[Anoop Saxena]

Broken. It will generate 0x2211ffaa, not the required
0x1122aaff.

Thanks. Stupid bug. :(

Anoop.

[Keith Thompson]

"ju**********@y ahoo.co.in" <ju**********@y ahoo.co.inwrite s:

I need to convert a big endian integer to little endian integer.
(the integer is 4 bytes in size on my implementation) . I came up with
the following code. I need your comments on this. Please suggest any
improvements that can be done.

#include <stdio.h>
int main(void)
{
int big = 0x12345678;
int little;
char *big_ptr = (char *)&big;
char *little_ptr = (char *)&little;

little_ptr[0] = big_ptr[3];
little_ptr[1] = big_ptr[2];
little_ptr[2] = big_ptr[1];
little_ptr[3] = big_ptr[0];

printf("big = 0x%x little = 0x%x\n",big,lit tle);

}

I'd use unsigned char rather than plain char. It's not likely to make
any real difference, but unsigned char is generally better for
arbitrary binary data.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <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.

[Stephen Sprunk]

"Anoop Saxena" <an**********@g mail.comwrote in message
news:11******** *************@t 46g2000cwa.goog legroups.com...

If int is 32 bits, then how good is the following code for unsigned
int's?

int main(void)
{
unsigned int x = 0xffaa2211;
unsigned int z = 0;
z = ( (x << 16) | ( x >16) );

return 0;
}

That converts little-endian to/from PDP-endian, not big-endian.

S

--
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking
--
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