getc() 作为宏和 C 标准库函数定义，连贯吗？

Question

在[7.1.4库函数的使用]中，我读到：

Any function declared in a header may be additionally implemented as a function-like macro defined in the header...

和

Any invocation of a library function that is implemented as a macro shall expand to code that evaluates each of its arguments exactly once...

然后对于getc,[7.21.7.5的getc函数]:

The getc function is equivalent to fgetc, except that if it is implemented as a macro, it may evaluate stream more than once, so the argument should never be an expression with side effects.

是否定义getc:

• 与库函数定义相矛盾？
• 反过来？
• 这是否与标准不一致？
• 或者这是否意味着如果 getc 单独实现（似乎不兼容但是？）作为一个宏它可能会对其参数求值两次？

Answer 1

的确，getc 宏可能会多次计算其 fp 参数。如果 §7.1.4 说 "Unless otherwise noted, any invocation of a library function that is implemented as a macro shall expand to code that evaluates each of its arguments exactly once."

可能会更好

getc 多次评估其 fp 参数的实现可以追溯到 stdio 的黎明。所以这并不奇怪，并且基本上没有依赖于单一评估或在多重评估下会中断的代码。 （谁写过像 getc(*fpp++) 这样的东西？是的，我可以想出一个例子，甚至不是 100% 人为的例子，但实际上，这种例子非常罕见。）

真正非常关心的代码总是可以调用 fgetc。这就是它的用途。

Answer 2

标准中的定义是连贯的；您对它们的尝试解释并不完全连贯。

标准说……

ISO/IEC 9899:2011 (C11) 标准说（从 §7.1.4 中引用了更多的 material ，并将一大段的部分内容分成几段）：

Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow: …

Any function declared in a header may be additionally implemented as a function-like macro defined in the header, so if a library function is declared explicitly when its header is included, one of the techniques shown below can be used to ensure the declaration is not affected by such a macro.

Any macro definition of a function can be suppressed locally by enclosing the name of the function in parentheses, because the name is then not followed by the left parenthesis that indicates expansion of a macro function name. For the same syntactic reason, it is permitted to take the address of a library function even if it is also defined as a macro.¹⁸⁵⁾ The use of #undef to remove any macro definition will also ensure that an actual function is referred to.

Any invocation of a library function that is implemented as a macro shall expand to code that evaluates each of its arguments exactly once, fully protected by parentheses where necessary, so it is generally safe to use arbitrary expressions as arguments.¹⁸⁶⁾ Likewise, those function-like macros described in the following subclauses may be invoked in an expression anywhere a function with a compatible return type could be called.¹⁸⁷⁾

¹⁸⁵⁾ This means that an implementation shall provide an actual function for each library function, even if it also provides a macro for that function.

¹⁸⁶⁾ Such macros might not contain the sequence points that the corresponding function calls do.

¹⁸⁷⁾ Because external identifiers and some macro names beginning with an underscore are reserved, implementations may provide special semantics for such names. For example, the identifier _BUILTIN_abs could be used to indicate generation of in-line code for the abs function. Thus, the appropriate header could specify

#define abs(x) _BUILTIN_abs(x)

for a compiler whose code generator will accept it. In this manner, a user desiring to guarantee that a given library function such as abs will be a genuine function may write

#undef abs

whether the implementation’s header provides a macro implementation of abs or a built-in implementation. The prototype for the function, which precedes and is hidden by any macro definition, is thereby revealed also.

请特别注意脚注 185 的内容。

您还从 §7.21.7.5 的 getc 的定义中引用了 material：

The getc function is equivalent to fgetc, except that if it is implemented as a macro, it may evaluate stream more than once, so the argument should never be an expression with side effects.

（其中 stream 是用于 getc 的参数的名称。）

解释标准

你问（略有转述）：

• getc的定义与库函数的定义是否矛盾？

  没有。 §7.1.4开头说了'unless explicitly stated otherwise'，然后给出了一系列的一般规则，然后getc的规范明确指出了其他的。

• 反之亦然吗？

  没有。 §7.1.4 的开头部分说任何特定函数的规范都可以覆盖 §7.1.4 中的一般性。

• 这是否与标准不一致？

  我看不出这里有任何不连贯之处。

• 或者这是否意味着如果 getc 仅作为宏实现（这似乎不符合要求，但是......），宏可能会评估其争论两次？

  1. getc 可能无法单独实现为宏（脚注 185）。还必须有一个实现相同功能的实际函数。实现可以很简单：

     int (getc)(FILE *fp) { return getc(fp); }
     

  2. 实现 getc 的宏被明确允许多次计算其参数（但不是必须这样做）。 §7.21.7.5 中的规范明确表示可以，而 §7.1.4 中的规范明确表示允许 §7.21.7.5 更改通常禁止此类行为的 §7.1.4 的一般规则。