[C/C++] Case study! Bad example about ignoring data-align..

Language/C&C++ 2007.07.09 22:23
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We should not access memory address directly to read the value.

For example,

Int8* pt;
malloc(pData, 6);
pt = pData;
*pt = length;
*pt = width;
*pt = (int)size; // ---(*)

This may create unexpected result due to data align. See (*). At (*), address of 'pt' is not 4-byte-aligned. But, it tries to write 4-byte-data (integer value). In this case, result may be compiler or target architecture dependent - usually, 'size' will be written at 4-byte-aligned-address.
Programmer who wrote above code may intent to make following memory state.

pData : [length(1 byte)][width(1 byte)][size(4 bytes]

But, usually, this may create following or undefined result.

pData : [length(1 byte)][width(1 byte)][padding(2 bytes)][size(4 bytes)]

So, we should not use like above(accessing memory directly). We can use 'struct' instead.
But, use 'memcpy' if above way is really needed.

(*) ==> memcpy(pt, &size, sizeof(size));

We should always keep it mind that data is aligned. So, directly accessing memory address to read or write data always has risk of compatibility with align.

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