结构的单字节对齐抵触架构的对齐要求？ [英] Struct one-byte alignment conflicted with alignment requirement of the architecture?

问题描述

我previously发布了一个问题，<一个href="http://stackoverflow.com/questions/13881487/should-i-worry-about-the-alignment-during-pointer-casting#comment19155923_13881487">here有关对齐时指针铸造的访问。作为总结，最好不要使用对齐访问是完全可移植的，因为有些建筑可能会引发异常，或者表现可能会非常慢比较一致的访问。

I previously posted a question here about aligned access during pointer casting. As a summary, it's better not to use unaligned access to be fully portable because some architecture may throw an exception, or the performance may get quite slower compared to aligned access.

然而，有的情况下，我想用一个字节对齐，例如， transfering网络数据时，我不想增加额外的补白内部结构。所以平时有什么这里的所做的是：

However, there are cases that I want to use one-byte alignment, e.g., during transfering network data, I don't want adding extra paddings inside structure. So usually what's done here is:

#pragma pack (push, 1)
struct tTelegram
{
   u8 cmd;
   u8 index;
   u16 addr1_16;
   u16 addr2_16;
   u8  length_low;
   u8 data[1];
};
#pragma pack (pop)

这时，你可能已经知道了我的问题：如果我执行的单字节对齐对我的结构，这是否意味着它不可能是完全可移植的，因为结构成员不对齐？如果我想既没有填充和便携性？

Then you might already know my question: If I enforce one-byte alignment on my struct, does that mean it cannot be fully portable, because struct members are not aligned? What if I want both no paddings and portability?

谢谢！

推荐答案

首先，未对齐的内存访问是指单件的数据跨越记忆多个单词。例如：在32位系统的32位 INT 地址0，4，8等对齐，但1，2，3，5，6， 7,9等将错位。

Firstly, misaligned memory accesses refers to single pieces of data that span multiple words in memory. For example: on a 32-bit system a 32-bit int at address 0, 4, 8 etc is aligned, but at 1, 2, 3, 5, 6, 7, 9 etc. would be misaligned.

其次，未对齐的数据并不抛出异常在C ++的观点，但可能会引发中断/陷阱/异常在CPU级别 - 例如， SIGBUS在UNIX上，在那里你会通常设置一个信号处理器来应对这一点，但如果你需要解析未对齐的数据在一个可移植的方式，你就不会被抓的信号做 - 你手动$ C C的步骤$打包和解包的数据跨越字边界。

Secondly, misaligned data doesn't "throw an exception" in the C++ sense, but may raise an interrupt/trap/exception at the CPU level - e.g. SIGBUS on UNIX, where you'd generally set a signal handler to react to this, but if you need to parse misaligned data in a portable way you wouldn't do so by catching signals - you'd manually code the steps to pack and unpack data spanning word boundaries.

在你的 tTelegram 结构，数据不错位，但位移位和屏蔽的数据，它的包装/解压缩从寄存器的过程中仍有可能慢 - 需要更多的机code指令 - 比使用数据占用一个独立的字

In your tTelegram struct, the data is not "misaligned", but the process of bit shifting and masking the data as it's packed/unpacked from a register is still likely slower - requiring more machine code instructions - than using data that occupies an independent word.

对于便携性 - 所有非玩具的编译器都会有一个选项，在你所描述的方式来包装，但具体编译会有所不同，在多字节值的字节的布局可能还是大端或little-端（或一些普通怪），而一些CPU允许一些未对齐的数据访问（如86）别人不一样（例如的Ultrasparc）。

Regarding portability - all non-toy compilers will have an option to pack in the way you've described, but the exact pragma will vary, the layout of bytes in multi-byte values may still be big-endian or little-endian (or something plain weird), and while some CPUs allow some misaligned data access (e.g. x86) others don't (e.g. Ultrasparc).

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