Initial commit

c++/Others/Byte alignment in a struct.cpp

@@ -0,0 +1,96 @@
+Byte alignment in a struct
+
+#include <iostream.h>
+
+#define ALIGN 8
+#pragma pack(ALIGN)
+
+/* Algorithm
+
+Byte offset between currentType and nextType  = MIN( multiple of Byte
+Alignment specified , multiple of size of nextType)
+**/
+
+int ALIGNOFFSET(int currRelativeAddess,int currrentTypeSize,int
+nextTypeSize)
+{
+
+	int nOffset = currrentTypeSize ;//this is size of the current
+type.Offset will be minimum of this size.
+	currRelativeAddess += currrentTypeSize ; //cuurent address is
+increased by size of current type.
+	if( nextTypeSize < ALIGN) // now alignment depens on the next type
+and also alignment defined by #pragam pack(#)
+	{
+		// if the size of next type is less than byte alignment size
+specified , then offset will be increased to the muliple of
+		// size of next type.
+		int nRemainder  = currRelativeAddess % nextTypeSize ;
+		if(nRemainder != 0)
+		{
+			nOffset += nextTypeSize - nRemainder ;//this will
+align boundary to next multiple of nextTypeSize
+		}
+
+	}
+	else
+	{
+		// if the byte alignment size specified is less than size of
+next type , then offset will be increased to the next muliple of
+		// size of byte alignment.
+		int nRemainder  = currRelativeAddess % ALIGN ;
+		if(nRemainder != 0)
+		{
+			nOffset += ALIGN - nRemainder ;//this will align the
+boundary to next multiple of ALIGN
+		}
+	}
+	return nOffset ;
+}
+
+
+struct S
+{
+
+   char a[2];
+   int b;
+   double c;
+   char d[1];
+   int e;
+   double f;
+};
+
+void main()
+{
+	S sz;
+	cout << "size of struct :" << sizeof(sz) << endl;
+	cout << "address of size::a "<< &sz.a <<" Actual offset = " <<
+int(0) << " Calculated offset = " << 0 << endl;
+
+	int offset = ((int)&sz.b)- (int)(&sz.a);
+
+
+	cout << "address of size::b " << &sz.b <<" Actual offset = " <<
+offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.a)-
+(int)(&sz),sizeof(sz.a),sizeof(sz.b)) << endl;
+	offset = ((int)&sz.c)- (int)(&sz.b);
+
+	cout << "address of size::c " << &sz.c <<" Actual offset = " <<
+offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.b) -
+(int)(&sz),sizeof(sz.b),sizeof(sz.c)) << endl;
+	offset = ((int)&sz.d)- (int)(&sz.c);
+
+	cout << "address of size::d " << &sz.d <<" Actual offset = " <<
+offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.c)-
+(int)(&sz),sizeof(sz.c),sizeof(sz.d)) << endl;
+
+	offset = ((int)&sz.e)- (int)(&sz.d);
+
+	cout << "address of size::e " << &sz.e <<" Actual offset = " <<
+offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.d)-
+(int)(&sz),sizeof(sz.d),sizeof(sz.e)) << endl;
+	offset = ((int)&sz.f)- (int)(&sz.e);
+	cout << "address of size::f " << &sz.f <<" Actual offset = " <<
+offset << " Calculated offset = " << ALIGNOFFSET((int)(&sz.e)-
+(int)(&sz),sizeof(sz.e),sizeof(sz.f)) << endl;
+}