_布尔类型和严格的别名 [英] _Bool type and strict aliasing

问题描述

我试图编写一些宏以供类型安全使用_Bool，然后对我的代码进行压力测试.出于邪恶测试的目的，我想出了这个肮脏的技巧:

I was trying to write some macros for type safe use of _Bool and then stress test my code. For evil testing purposes, I came up with this dirty hack:

_Bool b=0;
*(unsigned char*)&b = 42;

鉴于_Bool在实现sizeof(_Bool)==1上为1个字节)，我看不到这种黑客是如何违反C标准的.这不应该是严格的别名冲突.

Given that _Bool is 1 byte on the implementation sizeof(_Bool)==1), I don't see how this hack violates the C standard. It shouldn't be a strict aliasing violation.

但是通过各种编译器运行该程序时，我遇到了问题:

Yet when running this program through various compilers, I get problems:

#include <stdio.h>

int main(void)
{
  _Static_assert(sizeof(_Bool)==1, "_Bool is not 1 byte");

  _Bool b=0;
  *(unsigned char*)&b = 42;
  printf("%d ", b);
  printf("%d", b!=0 );

  return 0;
}

(代码依赖于printf隐式默认参数提升为int)

(The code relies on printf implicit default argument promotion to int)

某些版本的gcc和clang提供输出42 42，其他版本提供0 0.即使禁用优化.我本来期望42 1.

Some versions of gcc and clang give output 42 42, others give 0 0. Even with optimizations disabled. I would have expected 42 1.

似乎编译器假定_Bool只能是1或0，但同时在第一种情况下它愉快地打印42.

It would seem that the compilers assume that _Bool can only be 1 or 0, yet at the same time it happily prints 42 in the first case.

Q1:为什么?上面的代码是否包含未定义的行为?

Q1: Why is this? Does the above code contain undefined behavior?

Q2:sizeof(_Bool)的可靠性如何? C17 6.5.3.4完全没有提到_Bool.

Q2: How reliable is sizeof(_Bool)? C17 6.5.3.4 does not mention _Bool at all.

推荐答案

Q1:为什么?上面的代码是否包含未定义的行为?

Q1: Why is this? Does the above code contain undefined behavior?

是的，确实如此.存储有效，但随后读取为_Bool无效.

Yes, it does. The store is valid, but subsequently reading that as a _Bool is not.

6.2.6类型的表示形式

6.2.6.1常规

5某些对象表示形式不必表示对象类型的值.如果对象的存储值具有这种表示形式，并且由不具有字符类型的左值表达式读取，则该行为是不确定的. [...]

5 Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. [...]

Q2:sizeof(_Bool)的可靠性如何? C17 6.5.3.4完全没有提到_Bool.

Q2: How reliable is sizeof(_Bool)? C17 6.5.3.4 does not mention _Bool at all.

它将可靠地告诉您存储一个_Bool所需的字节数. 6.5.3.4也没有提到int，但是您不是在问sizeof(int)是否可靠，对吗?

It will reliably tell you the number of bytes that are needed to store one _Bool. 6.5.3.4 also doesn't mention int, but you're not asking whether sizeof(int) is reliable, are you?

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