是否可以在 C 中打印预处理器变量？

发布于 2024-07-29 19:39:32 字数 337 浏览 2 评论 0原文

是否可以将 C 中预处理器变量的值打印到 stderr？例如，我现在拥有的是：

#define PP_VAR (10)
#if (PP_VAR > 10)
    #warning PP_VAR is greater than 10
#endif

但我想做的是：

#define PP_VAR (10)
#if (PP_VAR > 10)
    #warning PP_VAR=%PP_VAR%
#endif

在C中可能有这样的事情吗？

原文

Is is possible to print to stderr the value of a preprocessor variable in C? For example, what I have right now is:

#define PP_VAR (10)
#if (PP_VAR > 10)
    #warning PP_VAR is greater than 10
#endif

But what I'd like to do is:

#define PP_VAR (10)
#if (PP_VAR > 10)
    #warning PP_VAR=%PP_VAR%
#endif

Is something like this possible in C?

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花期渐远 2024-08-05 19:39:32

您可以在 Visual Studio 下打印出预处理器变量的值。以下打印出 _MSC_VER 的值：

#define STRING2(x) #x
#define STRING(x) STRING2(x)

#pragma message(STRING(_MSC_VER))

但不确定这有多标准。

You can print out the value of a preprocessor variable under visual studio. The following prints out the value of _MSC_VER:

#define STRING2(x) #x
#define STRING(x) STRING2(x)

#pragma message(STRING(_MSC_VER))

Not sure how standard this is though.

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抱着落日 2024-08-05 19:39:32

这适用于 GCC 4.4.3：

#define STRING2(x) #x
#define STRING(x) STRING2(x)
#pragma message "LIBMEMCACHED_VERSION_HEX = " STRING(LIBMEMCACHED_VERSION_HEX)

产生：

src/_pylibmcmodule.c:1843: note: #pragma message: LIBMEMCACHED_VERSION_HEX = 0x01000017

This works with GCC 4.4.3:

#define STRING2(x) #x
#define STRING(x) STRING2(x)
#pragma message "LIBMEMCACHED_VERSION_HEX = " STRING(LIBMEMCACHED_VERSION_HEX)

yields:

src/_pylibmcmodule.c:1843: note: #pragma message: LIBMEMCACHED_VERSION_HEX = 0x01000017

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你的往事 2024-08-05 19:39:32

许多 C 编译器支持 #warning，但它不是由 C 标准定义的，除非您使用 C23 或更高版本。

然而，GCC 至少不会对后面的数据进行预处理，这意味着很难看到变量的值。

#define PP_VAR 123
#warning "Value of PP_VAR = " PP_VAR
#warning "Value of PP_VAR = " #PP_VAR
#warning "Value of PP_VAR = " ##PP_VAR

GCC 制定：

x.c:2:2: warning: #warning "Value of PP_VAR = " PP_VAR
x.c:3:2: warning: #warning "Value of PP_VAR = " #PP_VAR
x.c:4:2: warning: #warning "Value of PP_VAR = " ##PP_VAR

C23 标准预计会说明：

§6.10.6 诊断指令
语义
任一形式的预处理指令
# 错误 pp-tokensopt new-line 
  # 警告 pp-tokensopt 换行 
  
使实现产生一条诊断消息，其中包括指定的序列
预处理标记。

由于它指定诊断消息包含预处理标记，因此宏不会在 #error 或 #warning 的消息中展开。指定#error 的标准均未明确指定如果执行该指令，翻译单元的编译应失败 — 仅应生成诊断信息。然而，有一个合理的预期是，如果执行#error指令，TU应该无法编译。相比之下，执行 #warning 指令不应导致 TU 编译失败。

Many C compilers support #warning, but it is not defined by the C standard unless you're using C23 or later.

However, GCC at least does not do pre-processing on the data that follows, which means it is hard to see the value of a variable.

#define PP_VAR 123
#warning "Value of PP_VAR = " PP_VAR
#warning "Value of PP_VAR = " #PP_VAR
#warning "Value of PP_VAR = " ##PP_VAR

GCC produces:

x.c:2:2: warning: #warning "Value of PP_VAR = " PP_VAR
x.c:3:2: warning: #warning "Value of PP_VAR = " #PP_VAR
x.c:4:2: warning: #warning "Value of PP_VAR = " ##PP_VAR

The C23 standard is expected to say:

§6.10.6 Diagnostic directives
Semantics
A preprocessing directive of either form
# error pp-tokensopt new-line
# warning pp-tokensopt new-line
causes the implementation to produce a diagnostic message that includes the specified sequence of
preprocessing tokens.

Since it specifies that the diagnostic message includes the preprocessing tokens, macros will not be expanded in the message for either #error or #warning. None of the standards that specify #error explicitly specify that the compilation of the translation unit should fail if the directive is executed — only that a diagnostic shall be produced. Nevertheless, there is a reasonable expectation that if a #error directive is executed, the TU should fail to compile. By contrast, executing a #warning directive should not cause the compilation of the TU to fail.

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瑾兮 2024-08-05 19:39:32

使用预处理器标记粘贴运算符：##TOKEN_NAME

如前所述，您使用的预处理器指令是非标准的，所以 YMMV。

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醉态萌生 2024-08-05 19:39:32

好吧，你所做的实际上是不标准的。首先，#warning 或 #warn 指令不是标准的。其次，使用预处理器时，该行必须以井号开头，没有任何空格：

#ifdef BLAH1
#    define BLAH2 // OK, because pound is at the very left.
#endif

#ifdef BLAH3
     #define BLAH4 // Works on many compilers, but is non-standard.
#endif

由于您已经在使用非标准扩展，因此您需要查找您正在使用的特定预处理器/编译器的文档看看它是如何描述#warning的。

Well, what you are doing is actually non-standard. Firstly, the #warning or #warn directive is not standard. Secondly, when using the preprocessor, the line must begin with the pound symbol, without any spaces:

#ifdef BLAH1
#    define BLAH2 // OK, because pound is at the very left.
#endif

#ifdef BLAH3
     #define BLAH4 // Works on many compilers, but is non-standard.
#endif

Since you are already using a non-standard extension, you will need to look up the documentation of the particular preprocessor/compiler that you are using to see what it says about #warning.

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