字节对齐

字节对齐的细节和编译器实现相关，但一般而言，满足三个准则：
1) 结构体变量的首地址能够被其最宽基本类型成员的大小所整除；
2) 结构体每个成员相对于结构体首地址的偏移量（offset）都是成员大小的整数倍，如有需要编译器会在成员之间加上填充字节（internal adding）；
3) 结构体的总大小为结构体最宽基本类型成员大小的整数倍，如有需要编译器会在最末一个成员之后加上填充字节（trailing padding）。

Computing padding[edit]

The following formulas provide the number of padding bytes required to align the start of a data structure (where mod is the modulo operator):

# pseudo-code, see actual code below
padding = (align - (offset mod align)) mod align
new offset = offset + padding = offset + (align - (offset mod align)) mod align

For example, the padding to add to offset 0x59d for a structure aligned to every 4 bytes is 3. The structure will then start at 0x5a0, which is a multiple of 4. Note that when offset already is a multiple of align, the second modulo in (align - (offset mod align)) mod align is required to get a padding of 0.

If the alignment is a power of two, the modulo operation can be reduced to a bitwise boolean AND operation. The following formulas provide the new offset (where& is a bitwise AND and ~ a bitwise NOT):

padding = align - (offset & (align - 1)) = (-offset) & (align - 1)
new offset = (offset + align - 1) & ~(align - 1)