不对齐的内存访问总是导致总线错误？ [英] Does unaligned memory access always cause bus errors?

问题描述

据维基百科页面的 分段故障 的，总线错误可通过对齐的内存存取所引起的。本文提供了有关如何触发总线错误的例子。在这个例子中，我们要启用对齐检查，看看总线错误。如果我们禁用此类对齐检查？

该计划似乎正常工作。我有一个程序访问未对齐的内存频繁，它使用了不少人，但没有人报告总线错误或其它奇怪的结果给我。如果我们禁用对齐检查，什么是未对齐的内存的副作用？

平台：// EN：我在 86 /的 X86-64 。我也用在Mac上GCC -arch竞价排名编译它想我的程序，它正常工作。

解决方案

1. 这可能是显著慢访问未对齐的内存（如，几次慢）。




2. 并非所有平台甚至支持未对齐访问 - x86和x64做，但IA64（安腾）没有，例如：

。


3. 一个编译器可以的的emulate 的对齐访问（VC ++做的声明为 __对齐 ia64上的指针，例如） - 插入附加的检查，以检测不对齐的情况下，并且单独地跨在对准边界的对象的加载/存储部件。这比上本机支持它，但是平台对齐访问速度更慢。

According to the Wikipedia page Segmentation fault, a bus error can be caused by unaligned memory access. The article gives an example about how to trigger a bus error. In the example, we have to enable alignment checking to see the bus error. What if we disable such alignment checking?

The program seems to work properly. I have a program access unaligned memory frequently, and it is used by quite a few people, but no one reports bus errors or other weird results to me. If we disable alignment checking, what is the side effect of unaligned memory?

Platforms: I am working on x86/x86-64. I also tried my program by compiling it with "gcc -arch ppc" on a Mac and it works properly.

解决方案

1. It may be significantly slower to access unaligned memory (as in, several times slower).

2. Not all platforms even support unaligned access - x86 and x64 do, but ia64 (Itanium) does not, for example.

3. A compiler can emulate unaligned access (VC++ does that for pointers declared as __unaligned on ia64, for example) - by inserting additional checks to detect the unaligned case, and loading/storing parts of the object that straddle the alignment boundary separately. That is even slower than unaligned access on platforms which natively support it, however.

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