RE: How should I test endianness in C or C++?

Hi,
Try using quotes instead of arrows. But first, you might want to include it
as part of your project.
Cheers,
Joseph

-----Original Message-----
From: programmingblind-bounce@xxxxxxxxxxxxx
[mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of Delaunay
Christophe
Sent: Tuesday, July 20, 2010 5:44 AM
To: programmingblind@xxxxxxxxxxxxx
Subject: RE: How should I test endianness in C or C++?

Hi Martin and all,

Many thanks for your help.

In fact, what you noticed is exactly what I want to do. More precisely,
I want to print the bits of a number always beginning with the most
significant bit and ending with the least significant bit. In other
words, I want to always print the bits of an octet as if it was
represented in big endian.

This is the reason why, when my code notices that it should work in big
endian, the bitfields data structure defines bit b0 first, and when the
code notices that it works in little endian, it defines the bitfields
data structure with bit b7 first.

In practice, this artefact is useful for code which deals with network
protocols implemented in different platforms. The needed data structures
are defined in some special .h file and then, it is manipulated by a
code which is totally independent from endianness.

Now, it works under linux. I posted my request on another more
linux-dedicated list and some kind people told me that I forgot the
following line:

#include <endian.h>

I added the line in my code and, under linux, it now seems to work like
a charm.

However, I also would greatly need this piece of code to work under
windows.

Under this OS, I'm compiling with Visual C++ pro 2005. The compiler
complains that <endian.h> does not exist. What should I replace
"#include <endian.h>" with under windows please?

Many thanks. Have a nice day. Chris D

-----Original Message-----
From: programmingblind-bounce@xxxxxxxxxxxxx
[mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of Martin Slack
Sent: mardi 20 juillet 2010 13:19
To: programmingblind@xxxxxxxxxxxxx
Subject: Re: How should I test endianness in C or C++?

Hi Christophe,

  Although your two structs are ordered differently, the print routine
takes 
the bits in order b0 to b7 whatever the endianness of the machine.

  Corrections welcome,

  Martin


----- Original Message ----- 
From: "Delaunay Christophe" <christophe.delaunay@xxxxxxxxxxxxxxx>
To: <programmingblind@xxxxxxxxxxxxx>
Sent: Tuesday, July 20, 2010 8:28 AM
Subject: How should I test endianness in C or C++?


Hi all,

Here is my piece of code:
------ BEGINNING OF CODE ------
/**
 *
 * This small program tests and explains endianness.
 *
 */

/* --- The needed header files --- */
#include <stdio.h> /* for input and output */
#include <inttypes.h> /* special integer types */

/* --- Given definition --- */
#ifdef WIN32 /* windows-like OS assumed */
#define ATTRIBUTE
#else /* linux-like OS assumed */
#define ATTRIBUTE __attribute__((packed))
#endif /* windows or linux */

/* --- The byte divided into 8 bit fields --- */
typedef struct _bitfields {
#if __BYTE_ORDER == __BIG_ENDIAN
uint8_t b0:1;
uint8_t b1:1;
uint8_t b2:1;
uint8_t b3:1;
uint8_t b4:1;
uint8_t b5:1;
uint8_t b6:1;
uint8_t b7:1;
#elif __BYTE_ORDER == __LITTLE_ENDIAN
uint8_t b7:1;
uint8_t b6:1;
uint8_t b5:1;
uint8_t b4:1;
uint8_t b3:1;
uint8_t b2:1;
uint8_t b1:1;
uint8_t b0:1;
#endif /* big or little endian */
} ATTRIBUTE bitfields;

/* --- main --- */
int main(void) {
uint8_t val=0;
bitfields* bf = (bitfields*)(&val);
#if __BYTE_ORDER == __BIG_ENDIAN
printf("This machine works in big endian.\n");
#elif __BYTE_ORDER == __LITTLE_ENDIAN
printf("This machine works in little endian.\n");
#endif /* big or little endian */
printf("To understand how bits are ordered into a byte.\n");
printf("Give a value : ");
scanf("%hhu", &val);
while ( val != 0 ) {
uint8_t n = bf->b0&1;
printf("Given value = %d.\n", val);
printf("val=%d,",n);
n = bf->b1&1; printf("%d,",n);
n = bf->b2&1; printf("%d,",n);
n = bf->b3&1; printf("%d,",n);
n = bf->b4&1; printf("%d,",n);
n = bf->b5&1; printf("%d,",n);
n = bf->b6&1; printf("%d,",n);
n = bf->b7&1; printf("%d from b0 to b7.\n",n);
printf("Give another value : ");
scanf("%hhu", &val);
} /* at this time, value is 0 */

printf("Finished.\n");
return 0;
} /* main */
------ END OF CODE ------

Problem: When I run this program, here is what I obtain:
------ BEGINNING OF OUTPUT ------
[delaunayc@rennxlxrda013 TestEndian]$ ./TestByteOrder
This machine works in big endian.
To understand how bits are ordered into a byte.
Give a value : 1
Given value = 1.
val=1,0,0,0,0,0,0,0 from b0 to b7.
Give another value : 2
Given value = 2.
val=0,1,0,0,0,0,0,0 from b0 to b7.
Give another value : 3
Given value = 3.
val=1,1,0,0,0,0,0,0 from b0 to b7.
Give another value : 4
Given value = 4.
val=0,0,1,0,0,0,0,0 from b0 to b7.
Give another value : 128
Given value = 128.
val=0,0,0,0,0,0,0,1 from b0 to b7.
Give another value : 130
Given value = 130.
val=0,1,0,0,0,0,0,1 from b0 to b7.
Give another value : 131
Given value = 131.
val=1,1,0,0,0,0,0,1 from b0 to b7.
Give another value : 0
Finished.
[delaunayc@rennxlxrda013 TestEndian]$
------ END OF OUTPUT ------

As you can see, this machine says that it works in big endian but
presents the byte order in little endian.

Therefore, I suspect my #if test to be bad but what did I do wrong
please?

The machine is a PC running Fedora 12. I compiled the above code with
GCC version 4.4.3.
Many thanks in advance. Have a nice day. Chris D
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