Wikipedia - Data structure alignment
Typical alignment of C structs on x86
Data structure members are stored sequentially in a memory so that in the structure below the member Data1 will always precede Data2 and Data2 will always precede 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 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.