Data Padding

http://en.wikipedia.org/wiki/Data_structure_alignment

Data structure members are stored sequentially in memory so that, in the structure below,the member Data1 will always precede Data2; and Data2 will always precede Data3

struct MyData{    short Data1;    short Data2;    short Data3;};

If the type "short" is stored in two bytes of memory then each member of the data structure depicted above would be 2-byte aligned. Data1 would be at offset 0, Data2 at offset 2 and Data3 at offset 4. The size of this structure would be 6 bytes.

The type of each member of the structure usually has a default alignment, meaning that it will, unless otherwise requested by the programmer, be aligned on a pre-determined boundary. The following typical alignments are valid for compilers from Microsoft (Visual C++), Borland/CodeGear (C++Builder), Digital Mars (DMC) and GNU (GCC) when compiling for 32-bit x86:

• A char (one byte) will be 1-byte aligned.
• A short (two bytes) will be 2-byte aligned.
• An int (four bytes) will be 4-byte aligned.
• A long (four bytes) will be 4-byte aligned.
• A float (four bytes) will be 4-byte aligned.
• A double (eight bytes) will be 8-byte aligned on Windows and 4-byte aligned on Linux (8-byte with-malign-double compile time option).
• A long long (eight bytes) will be 8-byte aligned.
• A long double (ten bytes with C++Builder and DMC, eight bytes with Visual C++, twelve bytes with GCC) will be 8-byte aligned with C++Builder, 2-byte aligned with DMC, 8-byte aligned with Visual C++ and 4-byte aligned with GCC.
• Any pointer (four bytes) will be 4-byte aligned. (e.g.: char*, int*)

The only notable difference in alignment for a 64-bit system when compared to a 32-bit system is:

• A long (eight bytes) will be 8-byte aligned.
• A double (eight bytes) will be 8-byte aligned.
• A long double (eight bytes with Visual C++, sixteen bytes with GCC) will be 8-byte aligned with Visual C++ and 16-byte aligned with GCC.
• Any pointer (eight bytes) will be 8-byte aligned.

Some data types are dependent on the implementation.

Here is a structure with members of various types, totaling 8 bytes before compilation:

struct MixedData{    char Data1;    short Data2;    int Data3;    char Data4;};

After compilation the data structure will be supplemented with padding bytes to ensure a proper alignment for each of its members:

struct MixedData  /* After compilation in 32-bit x86 machine */{    char Data1; /* 1 byte */    char Padding1[1]; /* 1 byte for the following 'short' to be aligned on a 2 byte boundary assuming that the address where structure begins is an even number */    short Data2; /* 2 bytes */    int Data3;  /* 4 bytes - largest structure member */    char Data4; /* 1 byte */    char Padding2[3]; /* 3 bytes to make total size of the structure 12 bytes */};

#pragma pack

http://stackoverflow.com/questions/3318410/pragma-pack-effect