如何使用长于64位的位字段？ [英] How to work with bitfields longer than 64 bits?

问题描述

问题说的一切。

如果我有一个96位字段：

If I have this for a 96-bit field:

uint32_t flags[3]; //(thanks @jalf!)

我最好去访问这个，因为我的子字段可以位于32位边界（例如，从位29到35的字段）上。

How do I best go about accessing this, given that my subfields therein may lie over the 32-bit boundaries (eg. a field that runs from bit 29 to 35)?

我需要我的访问尽可能快，我不想将它们迭代为数组的32位元素。

I need my accesses to be as fast as possible, so I'd rather not iterate over them as 32-bit elements of an array.

推荐答案

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[This answer is valid for C (and by extension, for C++ as well).]

平台无关的方式是适当地应用位掩码和位移位

The platform-independent way is to apply bit-masks and bit-shifts as appropriate.

因此，您的字段从29到35（含）：

So to get your field from 29 to 35 (inclusive):

  (flags[1] & 0xF)        << 3
| (flags[0] & 0xE0000000) >> 29  // The bitmask here isn't really needed, but I like symmetry!

显然，你可以编写一个类/函数/宏来自动化。

Obviously, you could write a class/function/macro to automate this.

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