为什么#pragma 被视为预处理器指令? [英] Why is #pragma considered a preprocessor directive?

问题描述

我知道它以 # 开头，它遵循 预处理指令格式.但是预处理器真的关心它吗?#pragma pack、#pragma once 和我知道的所有其他指令都由编译器处理.this question 中的评论甚至指出 #pragma 指令在预处理阶段幸存下来.所以我的问题是:

I know it starts with #, and it follows the preprocessing directives format. But does the preprocessor really care about it? The #pragma pack, #pragma once, and all other directives I know, are all proceeded by compilers. A comment in this question even states that #pragma directive survives the pre-processing stage. So my questions are:

1. 既然 #pragma 是用于编译器的，为什么它被认为是一个预处理指令?仅仅是因为它以#开头吗?
2. 预处理器真的能用 #pragma 做一些事情吗?

1. Since #pragma is for compiler, why is it considered a preprocessing directive? Is it just because it starts with #?
2. Does preprocessor really do something with #pragma?

推荐答案

为什么 #pragma 被视为预处理器指令?

Why is #pragma considered a preprocessor directive?

因为C标准是这么说的.它在预处理指令一章中指定，C17 6.10.6.除此之外，该标准对于 #pragma 应该做什么故意非常模糊，因为整个目的是做一些特定于编译器的事情.或者，如果无法识别编译指示 - 忽略它.

Because the C standard says so. It is specified in the chapter preprocessing directives, C17 6.10.6. Other than that, the standard is intentionally very vague with what #pragma should do, since the whole purpose is to do something compiler-specific. Or in case the pragma isn't recognized - ignore it.

没有指定某个编译器如何在内部处理编译指示的内容.

How a certain compiler handles the contents of a pragma internally isn't specified.

某些编译指示显然需要进行预处理，特别是启用/禁用某些编译器行为的编译器行为，例如 #pragma warning ... 等.其中很多必须在预处理期间进行评估，否则编译器将不知道如何编译代码.

Some pragmas obviously need to be pre-processed, notably the kind that enables/disables certain compiler behavior like #pragma warning ... etc. Lots of them must be evaluated during pre-processing or the compiler won't know how to compile the code.

预处理器真的能用 #pragma 做些什么吗?

Does preprocessor really do something with #pragma?

是的，它在翻译阶段 4 中对其进行评估:执行预处理指令，扩展宏调用，并执行 _Pragma 一元运算符表达式."

Yes, it evaluates it in translation phase 4: "Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed."

请注意，将预处理器与编译器分开主要是一种理论模型.实际上，预处理器和编译器通常彼此紧密集成.

Please note that having a pre-processor separated from the compiler is mostly a theoretical model. In reality the pre-processor and compiler are often rather tightly integrated with each other.

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