为＆放大器;错误号合法的C？ [英] Is &errno legal C?

问题描述

每7.5，

[错误]扩展到修改lvalue175），有int类型，其价值是由几个库函数设置为正的错误编号。这是不确定的errno是一个宏或者与外部连接声明的标识符。如果宏定义是为了访问一个实际的对象pssed燮$ P $，或程序定义的名称错误号的标识符，行为是不确定的。

[errno] expands to a modifiable lvalue175) that has type int, the value of which is set to a positive error number by several library functions. It is unspecified whether errno is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual object, or a program defines an identifier with the name errno, the behavior is undefined.

175）的宏错误号不必是一个对象的标识符。它可能扩大到从一个函数调用产生修改的左值（例如，*错误号（））。

175) The macro errno need not be the identifier of an object. It might expand to a modifiable lvalue resulting from a function call (for example, *errno()).

这我不清楚这是否足以要求错误号不违反约束。 C语言有左值（如寄存器存储类变量，但只能是自动的让错误号不能被定义为这样的）该＆安培; 经营者违反约束

It's not clear to me whether this is sufficient to require that errno not be a constraint violation. The C language has lvalues (such as register-storage-class variables; however these can only be automatic so errno could not be defined as such) for which the & operator is a constraint violation.

如果＆放大器;错误号是合法的C，就是要不断它要求

If &errno is legal C, is it required to be constant?

推荐答案

所以§6.5.3.2p1指定

So §6.5.3.2p1 specifies

一元和放大器的操作;运营商应是一个功能标志，一个[]或目*运算符的结果，或者指定的对象不是一个位域，不使用寄存器存储类说明中声明的左值。

The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier.

这我觉得，可采取意味着＆放大器;左值是罚款的左值，是不是在这两个类别。正如你提到的，错误号不能使用寄存器存储类说明中声明，我认为（虽然我不是追引用现在检查），你不能有一个位域有型纯 INT

Which I think can be taken to mean that &lvalue is fine for any lvalue that is not in those two categories. And as you mentioned, errno cannot be declared with the register storage-class specifier, and I think (although am not chasing references to check right now) that you cannot have a bitfield that has type of plain int.

所以，我认为，规范要求＆放大器;（错误）是合法的C

So I believe that the spec requires &(errno) to be legal C.

如果＆放大器;错误号是合法的C，就是要不断它需要

If &errno is legal C, is it required to be constant?

据我了解，允许点的，一部分错误号来做个宏（它是在如glibc中的原因），就是允许它做个参考到线程本地存储，在这种情况下，它肯定不会是跨线程不变。我看不出有任何理由期待它必须是恒定的。只要值错误号保留规定的语义，我看不出有任何理由不正当的C库无法更改＆放大器;错误号来指不同的存储器地址上的程序的过程中 - 例如通过释放每次你设定的时间重新分配后备存储错误号。

As I understand it, part of the point of allowing errno to be a macro (and the reason it is in e.g. glibc) is to allow it to be a reference to thread-local storage, in which case it will certainly not be constant across threads. And I don't see any reason to expect it must be constant. As long as the value of errno retains the semantics specified, I see no reason a perverse C library could not change &errno to refer to different memory addresses over the course of a program -- e.g. by freeing and reallocating the backing store every time you set errno.

您可以想像维持在库设置的最后N errno值的环形缓冲区，并具有＆放大器;错误号总是指向最新的。我不认为这将是非常有用的，但我看不出它违反了规范的任何方式。

You could imagine maintaining a ring buffer of the last N errno values set by the library, and having &errno always point to the latest. I don't think it would be particularly useful, but I can't see any way it violates the spec.

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