Endian change & Byte Array -> Struct

This is a discussion on Endian change & Byte Array -> Struct within the C Programming forums, part of the General Programming Boards category; So, I recently ran into a problem where I have a byte array { 0, 196, 0, 0, 0, 42 ...

  1. #1
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    Endian change & Byte Array -> Struct

    So, I recently ran into a problem where I have a byte array { 0, 196, 0, 0, 0, 42 } and I need to copy that into a struct defined as:
    Code:
    typedef struct tdFOO {
      UINT16 ID;
      UINT32 RetVal;
      .
      .
      .
    } FOO;
    Also, the byte order in the byte array needs to be reversed. I'm communicating with a system with a different endianness, so I can't just memcpy the byte array into my structure. Also, when I did try (before I realized the bytes were out of order), the memcpy only populated the first structure variable (ID) and none of the others.

    What I actually need is for the byte array to look like: { 196, 0, 42, 0, 0, 0 }

    Which when copied into the structure would cause ID = 196 and RetVal to = 42

    The only way I can think to do it is to write a function that swaps the appropriate number of bytes based on the type of the struct variable where it will be assigned, and then do a memcpy of the bytes I just swapped directly into the struct variable.

    Then I just do that over, and over, and over again until I've copied every value.

    Thanks!

  2. #2
    Unregistered User Yarin's Avatar
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    You fail to mention what the sizes of the remaining tdFOO variables are.

    >> Then I just do that over, and over, and over again until I've copied every value.
    Or use a loop.

    You should convert your variables' endianess using hton and ntoh.
    hton(3)
    A class that doesn't overload all operators just isn't finished yet. -- SmugCeePlusPlusWeenie
    A year spent in artificial intelligence is enough to make one believe in God. -- Alan J. Perlis

  3. #3
    cas
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    the memcpy only populated the first structure variable (ID) and none of the others.
    This would likely be because of padding between the members of your struct. A compiler is free to insert padding anywhere in a struct (except for right at the beginning) in order to align members properly. On a 32-bit system, I would expect there to be 2 bytes of padding after your first member, for example, so that the next one is aligned on a 4-byte boundary.

    I don't see any particular need for a memcpy(). You can do something like:
    Code:
    whatever.ID = swap16(&memory[0]);
    whatever.RetVal = swap32(&memory[2]);
    ...
    If you have control over the other end, I absolutely agree with using the ntoh family of functions if they're available. They're designed precisely for what you're doing.

  4. #4
    Guest Sebastiani's Avatar
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    Something generic:

    Code:
    void* to_le( void* buf, size_t len )
    {
    	typedef unsigned char byte;
    	typedef byte* pbyte;
    	static size_t const val = 1, res = *( ( pbyte )&val ) == 0;
    	pbyte ptr = ( pbyte )buf, end = ( ptr + len ) - 1;
    	if( res )
    	{
    		for( ; ptr < end; ++ptr, --end )
    		{
    			*ptr ^= *end;
    			*end = *ptr ^ *end;
    			*ptr ^= *end;
    		}
    	}
    	return buf;
    }
    
    void* to_le16( void* buf )
    {
    	return to_le( buf, 2 );
    }
    
    void* to_le32( void* buf )
    {
    	return to_le( buf, 4 );
    }
    
    void* to_le64( void* buf )
    {
    	return to_le( buf, 8 );
    }
    Warning: untested code. =}

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