结构填充和包装 [英] Structure padding and packing

问题描述

考虑:

struct mystruct_A
{
   char a;
   int b;
   char c;
} x;

struct mystruct_B
{
   int b;
   char a;
} y;

结构体的大小分别为 12 和 8.

The sizes of the structures are 12 and 8 respectively.

这些结构是填充的还是压缩的?

Are these structures padded or packed?

什么时候进行填充或打包?

When does padding or packing take place?

推荐答案

Padding 将结构成员与自然"地址边界对齐 - 例如，int 成员将具有偏移量，在 32 位平台上为 mod(4) == 0.默认情况下填充是打开的.它将以下间隙"插入到您的第一个结构中:

Padding aligns structure members to "natural" address boundaries - say, int members would have offsets, which are mod(4) == 0 on 32-bit platform. Padding is on by default. It inserts the following "gaps" into your first structure:

struct mystruct_A {
    char a;
    char gap_0[3]; /* inserted by compiler: for alignment of b */
    int b;
    char c;
    char gap_1[3]; /* -"-: for alignment of the whole struct in an array */
} x;

Packing，另一方面防止编译器进行填充 - 这必须明确要求 - 在 GCC 下它是 __attribute__((__packed__))，所以以下内容:

Packing, on the other hand prevents compiler from doing padding - this has to be explicitly requested - under GCC it's __attribute__((__packed__)), so the following:

struct __attribute__((__packed__)) mystruct_A {
    char a;
    int b;
    char c;
};

将在 32 位架构上生成 6 大小的结构.

would produce structure of size 6 on a 32-bit architecture.

尽管如此 - 未对齐的内存访问在允许它的架构(如 x86 和 amd64)上速度较慢，并且在诸如 SPARC 的严格对齐架构上被明确禁止.

A note though - unaligned memory access is slower on architectures that allow it (like x86 and amd64), and is explicitly prohibited on strict alignment architectures like SPARC.

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