How to eliminate the bitmap difference in big endian and little endian?
 

Hi,

Such as I have following bitmap structure and assigned value:
struct bm
{
char high2bits:2;
char mid4bits:4;
char low2bits:2;
};
bm.high2bits = 3;
bm.mid4bits = 0;
bm.low2bits = 0;

The char value is 0x03 in little endian mode, and the char value is 0xc0 in big endian mode. How to eliminate the bitmap difference?


Thanks,
Eric

Re: How to eliminate the bitmap difference in big endian and littleendian?
 

"Eric J.Hu" <ehu@lucent.com> writes:
> Such as I have following bitmap structure and assigned value:
> struct bm
> {
> char high2bits:2;
> char mid4bits:4;
> char low2bits:2;
> };
> bm.high2bits = 3;
> bm.mid4bits = 0;
> bm.low2bits = 0;
>
> The char value is 0x03 in little endian mode, and the char value is 0xc0
> in big endian mode. How to eliminate the bitmap difference?

The only allowed types for bit-fields are int, signed int, unsigned
int (plain int may be either signed or unsigned in this context only)
and _Bool (C99 only). Your compiler may allow char bit-fields, but
it's non-portable. You probably want to use unsigned int.

Layout of bit-fields is implementation-specific. If you want
portability between little-endian and big-endian systems, don't use
bit-fields at all; use an unsigned integer type and manipulate the
bits yourself.

--
Keith Thompson (The_Other_Keith) kst-u@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.

Re: How to eliminate the bitmap difference in big endian and little endian?
 

"Eric J.Hu" <ehu@lucent.com> wrote

> Such as I have following bitmap structure and assigned value
> The char value is 0x03 in little endian mode, and the char value is 0xc0
> in big endian mode. How > to eliminate the bitmap difference?
>
You shouldn't normally need to care about the endianness of your host
machine.
You do need to care about the endianness of binary format files.
Never read or write a structure directly to a file that needs to be ported.
Write the functions write16le(int x, FILE *fp) to write a 16-bit
little-endian integer to a file, and so on. Write each field individually.
That way the code works whatever happens to the C compiler's internal
structure layout.

Re: How to eliminate the bitmap difference in big endian and little endian?
 

Don't use bit fields, and in general, don't rely on any bit/byte layout. If
you have a short, int or long, use them as whole, don't access them
partially through pointers to chars. Rather, extract the appropriate bit
from its bit position by means of a bit mask, e.g. to set a bit in an int at
position n:
myint |= 1<<n;
to reset it:
myint &= ~(1<<n);
to check it:
if (myint & (1<<n)) op1(); // if bit's set
else op2(); // if bit's clear

And don't save any nontext data (anything other than array of characters) to
a file using fwrite(), likewise don't read with fread() anything other than
characters from a file. And you should be OK and free from the endianness.
You may develop functions to decompose short, int and long into a series of
chars using the methods similar to the above, something like this:

int i;
FILE *f = ...;
unsigned x = ...;
for (i=0; i<sizeof(x); i++)
{
unsigned char c = x & UCHAR_MAX;
fwrite (&c, 1, 1, f);
x >>= CHAR_BIT;
}

the above will save the entire integer x in LSB first way regardless of the
CPU's endianness.
It's a good question whether or not I should use 8 instead of CHAR_BIT... I
know some odd system (a DSP) where char is 16-bit long, there, I believe,
the bits 8 through 15 are ignored when char is saved. Actually, no, they
shouldn't be ignored, but then again, the file is still a sequence of 8-bit
chars (the DSP saves files to the PC through JTAG). I'm not sure of this
particular thing as I usually saved raw data in 16-bit pieces (shorts), not
any kind of text...

HTH,
Alex

"Eric J.Hu" <ehu@lucent.com> wrote in message
news:deu5u9$qp7@netnews.net.lucent.com...
Hi,

Such as I have following bitmap structure and assigned value:
struct bm
{
char high2bits:2;
char mid4bits:4;
char low2bits:2;
};
bm.high2bits = 3;
bm.mid4bits = 0;
bm.low2bits = 0;

The char value is 0x03 in little endian mode, and the char value is 0xc0 in
big endian mode. How to eliminate the bitmap difference?


Thanks,
Eric