Any Method to Determine Endianness at Compile Time ?

[perry.yuan]

How could I determine the endianness of my compile environment at
compile time, instead of run time? I need a macro ("some_expressi on"),
i.e.

#if some_expression
#define TARGET_IS_LITTL E_ENDIAN
#else
#define TARGET_IS_BIG_E NDIAN

No way or some way? TIA.

[Richard Tobin]

Re: Any Method to Determine Endianness at Compile Time ?

In article <27c855f8-9938-49d6-9667-485205abfbc7@c1 9g2000prf.googl egroups.com>,
perry.yuan <perry.yuan@gma il.comwrote:

>How could I determine the endianness of my compile environment at
>compile time, instead of run time?

You can't write a compile-time expression to determine it, because
endianness is a property of representations , not values, and there
are no objects whose representations can be examined at compile
time.

One solution is to run a program at compile time that determines
the endianness and outputs a suitable #define to a file which
you then include.

-- Richard
--
:wq

[Ben Pfaff]

Re: Any Method to Determine Endianness at Compile Time ?

"perry.yuan " <perry.yuan@gma il.comwrites:

How could I determine the endianness of my compile environment at
compile time, instead of run time? I need a macro ("some_expressi on")

One way this is commonly done is to build in two stages. First,
compile and run a test program that tests for the
implementation' s endianness. Then use the test program's output
to configure macros to be defined while building the rest of the
program.
--
"I've been on the wagon now for more than a decade. Not a single goto
in all that time. I just don't need them any more. I don't even use
break or continue now, except on social occasions of course. And I
don't get carried away." --Richard Heathfield

[Eric Sosman]

Re: Any Method to Determine Endianness at Compile Time ?

perry.yuan wrote:

How could I determine the endianness of my compile environment at
compile time, instead of run time? I need a macro ("some_expressi on"),
i.e.
>
#if some_expression
#define TARGET_IS_LITTL E_ENDIAN
#else
#define TARGET_IS_BIG_E NDIAN
>
No way or some way? TIA.

This is Question 10.16 in the comp.lang.c Frequently
Asked Questions (FAQ) list <http://www.c-faq.com/>. You've
been around this newsgroup long enough to have seen the
FAQ mentioned several dozens of times; shame on you for
not bothering to read it.

--
Eric.Sosman@sun .com

[jacob navia]

Re: Any Method to Determine Endianness at Compile Time ?

perry.yuan wrote:

How could I determine the endianness of my compile environment at
compile time, instead of run time? I need a macro ("some_expressi on"),
i.e.
>
#if some_expression
#define TARGET_IS_LITTL E_ENDIAN
#else
#define TARGET_IS_BIG_E NDIAN
>
No way or some way? TIA.

#include <stdio.h>
int main(void)
{
union {
char c;
int i;
} u;
u.i = 0;
u.c = 1;

if (u.i == 1)
printf("little endian\n");
else
printf("big endian\n");
}

This printsd "little endian" in the intel processor,
"big endian" in the power pc. I think it should work.



--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique

http://www.cs.virginia.edu/~lcc-win32

[jacob navia]

Re: Any Method to Determine Endianness at Compile Time ?

jacob navia wrote:

perry.yuan wrote:

>How could I determine the endianness of my compile environment at
>compile time, instead of run time? I need a macro ("some_expressi on"),
>i.e.
>>
>#if some_expression
>#define TARGET_IS_LITTL E_ENDIAN
>#else
>#define TARGET_IS_BIG_E NDIAN
>>
>No way or some way? TIA.

>
#include <stdio.h>
int main(void)
{
union {
char c;
int i;
} u;
u.i = 0;
u.c = 1;
>
if (u.i == 1)
printf("little endian\n");
else
printf("big endian\n");
}
>
This printsd "little endian" in the intel processor,
"big endian" in the power pc. I think it should work.
>
>
>

Sorry, I missed the "preprocess or" part.


--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique

http://www.cs.virginia.edu/~lcc-win32

[Willem]

Re: Any Method to Determine Endianness at Compile Time ?

jacob wrote:
) perry.yuan wrote:
)How could I determine the endianness of my compile environment at
)compile time, instead of run time? I need a macro ("some_expressi on"),
^^^^^^^ ^^^

) int main(void)
) {
) union {
) char c;
) int i;
) } u;
) u.i = 0;
) u.c = 1;
)
) if (u.i == 1)
) printf("little endian\n");
) else
) printf("big endian\n");
) }

Isn't that a run time solution ?


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

[Ben Pfaff]

Re: Any Method to Determine Endianness at Compile Time ?

jacob navia <jacob@nospam.c omwrites:

perry.yuan wrote:

>How could I determine the endianness of my compile environment at
>compile time, instead of run time? I need a macro ("some_expressi on"),

>
This printsd "little endian" in the intel processor,
"big endian" in the power pc. I think it should work.

I don't think you actually read the OP's question.
--
Ben Pfaff

http://benpfaff.org

[Richard Tobin]

Re: Any Method to Determine Endianness at Compile Time ?

In article <slrnfu2lm3.1uh 2.willem@snail. stack.nl>,
Willem <willem@stack.n lwrote:

>) int main(void)
>) {
>) union {
>) char c;
>) int i;
>) } u;
>) u.i = 0;
>) u.c = 1;
>)
>) if (u.i == 1)
>) printf("little endian\n");
>) else
>) printf("big endian\n");
>) }

>Isn't that a run time solution ?

Not if you run it at compile time :-)

-- Richard
--
:wq

[jacob navia]

Re: Any Method to Determine Endianness at Compile Time ?

Richard Tobin wrote:

In article <slrnfu2lm3.1uh 2.willem@snail. stack.nl>,
Willem <willem@stack.n lwrote:
>

>) int main(void)
>) {
>) union {
>) char c;
>) int i;
>) } u;
>) u.i = 0;
>) u.c = 1;
>)
>) if (u.i == 1)
>) printf("little endian\n");
>) else
>) printf("big endian\n");
>) }

>

>Isn't that a run time solution ?

>
Not if you run it at compile time :-)
>
-- Richard

I sent a message recognizing my error around 30 seconds
after I sent the first answer.

Again:

Excuse, it was a mistake


--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique

http://www.cs.virginia.edu/~lcc-win32

[Morris Dovey]

Re: Any Method to Determine Endianness at Compile Time ?

"perry.yuan " wrote:

>
How could I determine the endianness of my compile environment at
compile time, instead of run time? I need a macro ("some_expressi on"),
i.e.
>
#if some_expression
#define TARGET_IS_LITTL E_ENDIAN
#else
#define TARGET_IS_BIG_E NDIAN
>
No way or some way? TIA.

Hmm. If you're willing to run another program before the compile,
you could run something like this from your compiler script or
makefile (although it need only be run once on any given target
machine):

/* Method of determining endian-ness is Jacob Navia's */
/* Adjust path and filename to suit your situation */

#include <stdio.h>
#include <stdlib.h>
int main(void)
{ FILE *fp;
union
{ char c;
int i;
} u;
u.i = 0;
u.c = 1;
if (fp = fopen("/usr/include/endian.h","w"))
{ fprintf(fp,"#de fine %S_ENDIAN\n",(u .i&1)?"LITTLE": "BIG");
fclose(fp);
return EXIT_SUCCESS;
}
return EXIT_FAILURE;
}

then in your application just

#include <endian.h>

#ifdef BIG_ENDIAN
/* Big endian code */
#else
/* Little endian code */
#endif

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

http://www.iedu.com/DeSoto/

[Robbie Hatley]

Re: Any Method to Determine Endianness at Compile Time ?


"perry.yuan " wrote:

How could I determine the endianness of my compile environment at
compile time, instead of run time?

Read the manuals for your CPU, operating system, and compiler.

I need a macro

You could use a macro to do conditional compilation, yes. Your compiler
might even provide you with endian-ness macros.

But why not do the checking at run-time instead? Your code would be
more portable that way. Read Jacob Navia's reply to your post.
I'd recommend that approach.

Another method I've seen is, use a char pointer to point to the 0th
byte of an int which is set to 65. If the char pointed to is "A",
you're little endian. If it's the NUL character instead, you're
big-endian.

--
Cheers,
Robbie Hatley
lonewolf aatt well dott com
www dott well dott com slant user slant lonewolf slant

[Richard Tobin]

Re: Any Method to Determine Endianness at Compile Time ?

In article <Gs6dnfgoF6415H zanZ2dnUVZ_j6dn Z2d@giganews.co m>,
Robbie Hatley <see.my.signatu re@for.my.email .addresswrote:

>How could I determine the endianness of my compile environment at
>compile time, instead of run time?

>Read the manuals for your CPU, operating system, and compiler.

This is a stupid, unhelpful answer. The aim is obviously to produce a
system that works on systems you don't necessarily have.

>But why not do the checking at run-time instead?

That is often sufficient, but is sometimes unacceptably inefficient
(or verbose, if you replicate large chunks of code). If efficiency is
important, arrange to run a program at compile time to determine the
answer.

>Another method I've seen is, use a char pointer to point to the 0th
>byte of an int which is set to 65. If the char pointed to is "A",
>you're little endian.

Why confuse the issue by bringing in ASCII codes?

-- Richard
--
:wq

[CBFalconer]

Re: Any Method to Determine Endianness at Compile Time ?

Richard Tobin wrote:

Willem <willem@stack.n lwrote:
>

>>int main(void) {
>> union {
>> char c;
>> int i;
>> } u;
>> u.i = 0;
>> u.c = 1;
>>>
>> if (u.i == 1)
>> printf("little endian\n");
>> else
>> printf("big endian\n");
>>}

>

>Isn't that a run time solution ?

>
Not if you run it at compile time :-)

On the cross-compiler !!

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.


--
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[Richard]

Re: Any Method to Determine Endianness at Compile Time ?

"Robbie Hatley" <see.my.signatu re@for.my.email .addresswrites:

"perry.yuan " wrote:
>

>How could I determine the endianness of my compile environment at
>compile time, instead of run time?

>
Read the manuals for your CPU, operating system, and compiler.
>

>I need a macro

>
You could use a macro to do conditional compilation, yes. Your compiler
might even provide you with endian-ness macros.
>
But why not do the checking at run-time instead? Your code would be
more portable that way. Read Jacob Navia's reply to your post.
I'd recommend that approach.
>
Another method I've seen is, use a char pointer to point to the 0th
byte of an int which is set to 65. If the char pointed to is "A",
you're little endian. If it's the NUL character instead, you're
big-endian.

And the reason you can't use an int or long of value 1 is?