endian

[Marc Schellens]

Is there a standard conforming way to check for the endian of
the current machine? Possibly at compile time?

I am thinking of something like:

union endian
{
char c[2];
unsigned int i;
};

endian e; // can it be initialized directly?
e.i=1;

bool BigEndian()
{
return (e.c[0] == 0);
}

But this is not at compile time.
Further:
Is it guaranteed:
that an int has at least two bytes?
that the bit pattern for zero and one are the same
for char and int?

thanks,
marc

[Bob Hairgrove]

On Fri, 11 Jul 2003 10:55:49 +0900, Marc Schellens
<m_*********@hotmail.com> wrote:

Is there a standard conforming way to check for the endian of
the current machine? Possibly at compile time?

I am thinking of something like:

union endian
{
char c[2];
unsigned int i;
};

endian e; // can it be initialized directly?
e.i=1;

bool BigEndian()
{
return (e.c[0] == 0);
}

But this is not at compile time.
No, AFAIK standard C++ does not deal with big endian/little endian
issues. Most applications which need to support both (e.g. parsing of
TIFF image files) will rely on the "magic numbers" in the header of
such files and react accordingly.

Have you checked the Boost libraries? Perhaps there is something in
there.

Further:
Is it guaranteed:
that an int has at least two bytes?
No. An "int" has to be at least large enough for "the natural size for
integer arithmetic on a given machine".

There is also no standard definition of the number of bits in a
"byte". There is a standard header file <limits> which has constants
such as CHAR_BIT (or CHAR_BITS, I forgot which is correct), INT_MIN,
INT_MAX, SHRT_MAX etc. which might help you wite portable code. These
provide information consistent with the platform on which the
application has been compiled, however; if you need the information at
run-time, you'll need to do something OS-specific.
that the bit pattern for zero and one are the same
for char and int?

Since char is also an integral type, they would have to be (except for
the number of bits). Otherwise the shift operators '<<', '>>' and
binary operators '&', '^' etc. wouldn't work consistently.
--
Bob Hairgrove
rh**************@Pleasebigfoot.com

[Ivan Vecerina]

"Marc Schellens" <m_*********@hotmail.com> wrote in message
news:3F**************@hotmail.com...
| Is there a standard conforming way to check for the endian of
| the current machine? Possibly at compile time?

There is no way to check for endianess at compile time
- except using vendor-specific preprocessor identifiers.

For the runtime test, your code is ok if you don't want
to bother with exotic systems with mixed endianness etc.

union endian
{
char c[2];
unsigned int i;
};

endian e; // can it be initialized directly?
e.i=1;

bool BigEndian()
{
return (e.c[0] == 0);
}

This could be simplified into:
bool BigEndian()
{
unsigned short s = 0x0001;
return ! *(unsigned char*)&s;
}
Regards,
--
Ivan Vecerina, Dr. med. <> http://www.post1.com/~ivec
Soft Dev Manger, xitact <> http://www.xitact.com
Brainbench MVP for C++ <> http://www.brainbench.com