在 c 中为相同的标识符重用#define 是否错误？

Question

K&R 书作者说

A second #define for the same identifier is erroneous unless the second token sequence is identical to the first.

但是当我尝试时，编译器没有引发任何错误。那么，能否请您详细说明作者想表达的意思？

Answer 1

如果您按照您所说的去做，它非常应该引发诊断消息。从相关部分，C11 6.10.3 Macro replacement:

Two replacement lists are identical if and only if the preprocessing tokens in both have the same number, ordering, spelling, and white-space separation, where all white-space separations are considered identical.

An identifier currently defined as an object-like macro shall not be redefined by another #define preprocessing directive unless the second definition is an object-like macro definition and the two replacement lists are identical. Likewise, an identifier currently defined as a function-like macro shall not be redefined by another #define preprocessing directive unless the second definition is a function-like macro definition that has the same number and spelling of parameters, and the two replacement lists are identical.

例如，请参阅以下抄本：

pax:/paxhome> cat prog.c
#define X 7
#define X 99

int main(void) { return 99; }

pax:/paxhome> gcc --std=c17 prog.c -o prog
prog.c:2: warning: "X" redefined
    2 | #define X 99
      |
prog.c:1: note: this is the location of the previous definition
    1 | #define X 7
      |

请记住第一段中给出的“相同”的定义，它们不必在逐个字符的基础上完全相同。根据给出的规则，以下两个可以认为是相同的：

#define X 7 + 2
#define X \
    7 \
        + \
            2

另请记住，gcc 将此视为警告而非错误。该标准本身仅声明必须为无效程序生成诊断消息，但还包含以下脚注（我强调）：

Of course, an implementation is free to produce any number of diagnostics as long as a valid program is still correctly translated. It may also successfully translate an invalid program.

所以 gcc 有权简单地警告你，但无论如何都要编译程序。如果您想将警告视为错误（我倾向于将其用于我自己的代码），您始终可以将 -Werror 标志添加到编译命令（或者可能 -Wpedantic -pedantic-errors，具体取决于您的需要).

在 c 中为相同的标识符重用#define 是否错误？

Is it erroneous to reuse #define for the same identifier in c?

c

macros

kernighan-and-ritchie