97 lines
2.7 KiB
C++
97 lines
2.7 KiB
C++
Byte alignment in a struct
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#include <iostream.h>
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#define ALIGN 8
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#pragma pack(ALIGN)
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/* Algorithm
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Byte offset between currentType and nextType = MIN( multiple of Byte
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Alignment specified , multiple of size of nextType)
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**/
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int ALIGNOFFSET(int currRelativeAddess,int currrentTypeSize,int
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nextTypeSize)
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{
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int nOffset = currrentTypeSize ;//this is size of the current
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type.Offset will be minimum of this size.
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currRelativeAddess += currrentTypeSize ; //cuurent address is
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increased by size of current type.
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if( nextTypeSize < ALIGN) // now alignment depens on the next type
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and also alignment defined by #pragam pack(#)
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{
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// if the size of next type is less than byte alignment size
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specified , then offset will be increased to the muliple of
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// size of next type.
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int nRemainder = currRelativeAddess % nextTypeSize ;
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if(nRemainder != 0)
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{
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nOffset += nextTypeSize - nRemainder ;//this will
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align boundary to next multiple of nextTypeSize
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}
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}
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else
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{
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// if the byte alignment size specified is less than size of
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next type , then offset will be increased to the next muliple of
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// size of byte alignment.
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int nRemainder = currRelativeAddess % ALIGN ;
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if(nRemainder != 0)
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{
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nOffset += ALIGN - nRemainder ;//this will align the
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boundary to next multiple of ALIGN
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}
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}
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return nOffset ;
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}
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struct S
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{
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char a[2];
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int b;
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double c;
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char d[1];
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int e;
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double f;
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};
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void main()
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{
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S sz;
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cout << "size of struct :" << sizeof(sz) << endl;
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cout << "address of size::a "<< &sz.a <<" Actual offset = " <<
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int(0) << " Calculated offset = " << 0 << endl;
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int offset = ((int)&sz.b)- (int)(&sz.a);
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cout << "address of size::b " << &sz.b <<" Actual offset = " <<
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offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.a)-
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(int)(&sz),sizeof(sz.a),sizeof(sz.b)) << endl;
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offset = ((int)&sz.c)- (int)(&sz.b);
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cout << "address of size::c " << &sz.c <<" Actual offset = " <<
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offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.b) -
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(int)(&sz),sizeof(sz.b),sizeof(sz.c)) << endl;
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offset = ((int)&sz.d)- (int)(&sz.c);
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cout << "address of size::d " << &sz.d <<" Actual offset = " <<
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offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.c)-
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(int)(&sz),sizeof(sz.c),sizeof(sz.d)) << endl;
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offset = ((int)&sz.e)- (int)(&sz.d);
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cout << "address of size::e " << &sz.e <<" Actual offset = " <<
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offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.d)-
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(int)(&sz),sizeof(sz.d),sizeof(sz.e)) << endl;
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offset = ((int)&sz.f)- (int)(&sz.e);
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cout << "address of size::f " << &sz.f <<" Actual offset = " <<
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offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.e)-
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(int)(&sz),sizeof(sz.e),sizeof(sz.f)) << endl;
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}
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