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Visual C++ invalid sizeof or not ???

Posted on 2002-05-07
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Last Modified: 2013-12-14
Is this bug or not ????

DWORD test
struct stest
{
DWORD v1;
WORD v2;
}

test = sizeof(stest)

Variable test must be equial to 6 but it returns 8
????????????????
Waiting for Answer :)


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Question by:chep
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4 Comments
 
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jkr earned 100 total points
ID: 6994587
8 is correct, this is because of 'structure member alignment' - the compiler will add two padding bytes, so pointers to the strct are aligned on a DWORD boundary. To turn off that behaviour, use

#pragma pack ( 1)
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by:jkr
ID: 6994595
BTW, see also the chapter 'sizeof' on the VC++ online docs:

sizeof Operator
sizeof expression

The sizeof keyword gives the amount of storage, in bytes, associated with a variable or a type (including aggregate types). This keyword returns a value of type size_t.

The expression is either an identifier or a type-cast expression (a type specifier enclosed in parentheses).

When applied to a structure type or variable, sizeof returns the actual size, which may include padding bytes inserted for alignment. When applied to a statically dimensioned array, sizeof returns the size of the entire array. The sizeof operator cannot return the size of dynamically allocated arrays or external arrays.


Example

// Example of the sizeof keyword
size_t  i = sizeof( int );

struct align_depends {
    char c;
    int i;
};
size_t size = sizeof(align_depends);  // The value of size depends on
                                   //  the value set with /Zp or
                                   //  #pragma pack

int  array[] = { 1, 2, 3, 4, 5 };     // sizeof( array ) is 20
                                      // sizeof( array[0] ) is 4
size_t  sizearr =                        // Count of items in array
   sizeof( array ) / sizeof( array[0] );
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Expert Comment

by:thienpnguyen
ID: 6994598
It is not a bug. It uses algnment structure for fast access.
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LVL 86

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by:jkr
ID: 6994720
Thnx - but why a 'B'? Is there anything I left open?
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