Binary File - Byte order / endian

This is a discussion on Binary File - Byte order / endian within the C Programming forums, part of the General Programming Boards category; I have to write a binary file. The data is supposed to be written in Little Endian format, which is ...

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    Wanabe Laser Engineer chico1st's Avatar
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    Binary File - Byte order / endian

    I have to write a binary file.
    The data is supposed to be written in Little Endian format, which is a type of byte order.
    Does anyone know how to do this?

    Glossary:

    byte order:
    byte order sets the endian form of the resulting data. Byte order, or endian form, indicates whether integers are represented in memory from most-significant byte to least-significant byte or vice versa.
    little-endian—The least-significant byte occupies the lowest memory address. Used on Windows and Linux.

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    Yes, I know how to do that. [That is the question you asked! :-)]

    Do you have any thoughts on the matter?

    --
    Mats

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    Ok, I'll be kind and give a hint: Try finding out what byte-order your machine is...

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    Your reference says
    little-endian—The least-significant byte occupies the lowest memory address. Used on Windows and Linux.
    The part that says "Used in Windows and Linux" is true - just like "dogs have a tail" - doesn't mean that all animals with a tail are dogs, right?

    I don't know about Windows, but Linux can and does definitely run on big-endian machines, no doubt at all about that.

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    Guest Sebastiani's Avatar
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    first you need to determine the byte ordering of the machine your program is running under.
    a simple way to determine this would be:

    Code:
    int
    is_big_endian_machine(void)
    {
        static
        unsigned
        tst = 1;
    
        static
        int
        stat = (*(unsigned char*)&tst) != 1;
    
        return stat;
    }
    if it's big endian, simply reverse the bytes of each 'native' data type.
    the easiest way to do that is to cast the address of the data to an unsigned char* and then work with the data as an array of bytes.



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    Just Lurking Dave_Sinkula's Avatar
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    Isn't it also possible to do it portably without caring about a particular implementation's underlying multibyte storage? That is, where you truncate to a byte some integral value to get the least significant byte to put to a file, and then right-shift the value by CHAR_BIT; doing so for each byte in the object.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    Registered User MacNilly's Avatar
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    Quote Originally Posted by chico1st View Post
    I have to write a binary file.
    The only problem I can see is if you wanted to move a file written from one type of machine to the other. Guess that's why you can't move files from Macs to PCs.

    You want to try and convert a file from one to the other? Unless you know the specific data types that make up the file, that would be impossible.

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    Wanabe Laser Engineer chico1st's Avatar
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    so to switch the byte order my code would look like this:

    Code:
    buffer1 = (unsigned char *)OldNumber;
    bit[0] = find the first bit;
    bit[1] = find the second bit
    etc...
    bit[8] = ...
    
    sprintf(Buffer2, "%c%c%c%c%c%c%c%c", bit[0], bit[1],...);
    NewNumber = (unsigned short)Buffer2;
    or something to that extent?

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by MacNilly View Post
    The only problem I can see is if you wanted to move a file written from one type of machine to the other. Guess that's why you can't move files from Macs to PCs.
    Yet more thigh-slapping hilarity.

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    Deathray Engineer MacGyver's Avatar
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    Quote Originally Posted by brewbuck View Post
    Yet more thigh-slapping hilarity.
    Yeah, I kind of wondered about that.

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    little/big endian is processor related, not OS

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    Quote Originally Posted by KIBO View Post
    little/big endian is processor related, not OS
    At least to a part, hence my comment:
    The part that says "Used in Windows and Linux" is true - just like "dogs have a tail" - doesn't mean that all animals with a tail are dogs, right?

    I don't know about Windows, but Linux can and does definitely run on big-endian machines, no doubt at all about that.
    But at the same time, there are several processors I know of (Mips, 29K) that can actually set the byte-ordering by changing a bit in a control-register. I think ARM can do this too. In this case, the OS can decide what byte-ordering to use.

    --
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    Guest Sebastiani's Avatar
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    >> or something to that extent?

    consider the 4 byte hexidecimal number 0xbebafeca. on a big endian machine this would be stored in contigiuos memory as [be][ba][fe][ca], whereas on a little endian machine the format would be [ca][fe][ba][be]. viewed as an array of unsigned char, to reverse the byte order you would need simply to swap the first element of the array with the last and the second with the third.



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    Wanabe Laser Engineer chico1st's Avatar
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    oh, phew i thought i had to switch every single bit.

    so if i had 'unsigned shorts' (the data type), would i literally take the number lets say 16, which is 00000000 00010000 in binary, little endian format would be 00010000 00000000, which in integer is 4096. Then write that number to the binary file i have to write?

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    Wanabe Laser Engineer chico1st's Avatar
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    ok i googled the topic and i found a very simple code to swap these endians .. but it is in C++ therefore i dont know whats going on.

    Can you help me to convert this to C?

    Code:
    unsigned short ByteSwap4 (unsigned short nValue)
    {
       return (((nValue>> 8)) | (nValue << 8));
    
    }
    its from here.. and there is a little story to go with the artical
    http://www.codeproject.com/cpp/endianness.asp
    Last edited by chico1st; 08-22-2007 at 10:00 AM.

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