Standard Predefined Macros

本文介绍了C标准中预定义的宏,包括__DATE__、__FILE__、__LINE__等,这些宏可以帮助开发者获取编译时间、文件名、行号等信息,对于调试和版本控制非常有用。

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3.7.1 Standard Predefined Macros

The standard predefined macros are specified by the relevant language standards, so they are available with all compilers that implement those standards. Older compilers may not provide all of them. Their names all start with double underscores.

__FILE__
This macro expands to the name of the current input file, in the form of a C string constant. This is the path by which the preprocessor opened the file, not the short name specified in ` #include ' or as the input file name argument. For example, "/usr/local/include/myheader.h" is a possible expansion of this macro.
__LINE__
This macro expands to the current input line number, in the form of a decimal integer constant. While we call it a predefined macro, it's a pretty strange macro, since its “definition” changes with each new line of source code.

__FILE__ and __LINE__ are useful in generating an error message to report an inconsistency detected by the program; the message can state the source line at which the inconsistency was detected. For example,

     fprintf (stderr, "Internal error: "

"negative string length "

"%d at %s, line %d.",

length, __FILE__, __LINE__);

An `#include ' directive changes the expansions of __FILE__ and __LINE__ to correspond to the included file. At the end of that file, when processing resumes on the input file that contained the `#include ' directive, the expansions of __FILE__ and __LINE__ revert to the values they had before the `#include ' (but __LINE__ is then incremented by one as processing moves to the line after the `#include ').

A `#line ' directive changes __LINE__ , and may change __FILE__ as well. See Line Control .

C99 introduces __func__ , and GCC has provided __FUNCTION__ for a long time. Both of these are strings containing the name of the current function (there are slight semantic differences; see the GCC manual). Neither of them is a macro; the preprocessor does not know the name of the current function. They tend to be useful in conjunction with __FILE__ and __LINE__ , though.

__DATE__
This macro expands to a string constant that describes the date on which the preprocessor is being run. The string constant contains eleven characters and looks like "Feb 12 1996" . If the day of the month is less than 10, it is padded with a space on the left.

If GCC cannot determine the current date, it will emit a warning message (once per compilation) and __DATE__ will expand to "??? ?? ????" .

__TIME__
This macro expands to a string constant that describes the time at which the preprocessor is being run. The string constant contains eight characters and looks like "23:59:01" .

If GCC cannot determine the current time, it will emit a warning message (once per compilation) and __TIME__ will expand to "??:??:??" .

__STDC__
In normal operation, this macro expands to the constant 1, to signify that this compiler conforms to ISO Standard C. If GNU CPP is used with a compiler other than GCC, this is not necessarily true; however, the preprocessor always conforms to the standard unless the -traditional-cpp option is used.

This macro is not defined if the -traditional-cpp option is used.

On some hosts, the system compiler uses a different convention, where __STDC__ is normally 0, but is 1 if the user specifies strict conformance to the C Standard. CPP follows the host convention when processing system header files, but when processing user files __STDC__ is always 1. This has been reported to cause problems; for instance, some versions of Solaris provide X Windows headers that expect __STDC__ to be either undefined or 1. See Invocation .

__STDC_VERSION__
This macro expands to the C Standard's version number, a long integer constant of the form yyyy mm L where yyyy and mm are the year and month of the Standard version. This signifies which version of the C Standard the compiler conforms to. Like __STDC__ , this is not necessarily accurate for the entire implementation, unless GNU CPP is being used with GCC.

The value 199409L signifies the 1989 C standard as amended in 1994, which is the current default; the value 199901L signifies the 1999 revision of the C standard. Support for the 1999 revision is not yet complete.

This macro is not defined if the -traditional-cpp option is used, nor when compiling C++ or Objective-C.

__STDC_HOSTED__
This macro is defined, with value 1, if the compiler's target is a hosted environment . A hosted environment has the complete facilities of the standard C library available.
__cplusplus
This macro is defined when the C++ compiler is in use. You can use __cplusplus to test whether a header is compiled by a C compiler or a C++ compiler. This macro is similar to __STDC_VERSION__ , in that it expands to a version number. A fully conforming implementation of the 1998 C++ standard will define this macro to 199711L . The GNU C++ compiler is not yet fully conforming, so it uses 1 instead. It is hoped to complete the implementation of standard C++ in the near future.
__OBJC__
This macro is defined, with value 1, when the Objective-C compiler is in use. You can use __OBJC__ to test whether a header is compiled by a C compiler or an Objective-C compiler.
__ASSEMBLER__
This macro is defined with value 1 when preprocessing assembly language.


=================================================================

一、标准预定义宏
The standard predefined macros are specified by the relevant language standards, so they are available with all compilers that implement those standards. Older compilers may not provide all of them. Their names all start with double underscores.

__FILE__
This macro expands to the name of the current input file, in the form of a C string constant. This is the path by which the preprocessor opened the file, not the short name specified in #include or as the input file name argument. For example, "/usr/local/include/myheader.h" is a possible expansion of this macro.

__LINE__
This macro expands to the current input line number, in the form of a decimal integer constant. While we call it a predefined macro, it's a pretty strange macro, since its "definition" changes with each new line of source code.
__FILE__ and __LINE__ are useful in generating an error message to report an inconsistency detected by the program; the message can state the source line at which the inconsistency was detected. For example,

     fprintf (stderr, "Internal error: "
                      "negative string length "
                      "%d at %s, line %d.",
              length, __FILE__, __LINE__);
    
An #include directive changes the expansions of __FILE__ and __LINE__ to correspond to the included file. At the end of that file, when processing resumes on the input file that contained the #include directive, the expansions of __FILE__ and __LINE__ revert to the values they had before the #include (but __LINE__ is then incremented by one as processing moves to the line after the #include).

A #line directive changes __LINE__, and may change __FILE__ as well. See Line Control.

C99 introduces __func__, and GCC has provided __FUNCTION__ for a long time. Both of these are strings containing the name of the current function (there are slight semantic differences; see the GCC manual). Neither of them is a macro; the preprocessor does not know the name of the current function. They tend to be useful in conjunction with __FILE__ and __LINE__, though.


__DATE__
This macro expands to a string constant that describes the date on which the preprocessor is being run. The string constant contains eleven characters and looks like "Feb 12 1996". If the day of the month is less than 10, it is padded with a space on the left.
If GCC cannot determine the current date, it will emit a warning message (once per compilation) and __DATE__ will expand to "??? ?? ????".


__TIME__
This macro expands to a string constant that describes the time at which the preprocessor is being run. The string constant contains eight characters and looks like "23:59:01".
If GCC cannot determine the current time, it will emit a warning message (once per compilation) and __TIME__ will expand to "??:??:??".


__STDC__
In normal operation, this macro expands to the constant 1, to signify that this compiler conforms to ISO Standard C. If GNU CPP is used with a compiler other than GCC, this is not necessarily true; however, the preprocessor always conforms to the standard unless the -traditional-cpp option is used.
This macro is not defined if the -traditional-cpp option is used.

On some hosts, the system compiler uses a different convention, where __STDC__ is normally 0, but is 1 if the user specifies strict conformance to the C Standard. CPP follows the host convention when processing system header files, but when processing user files __STDC__ is always 1. This has been reported to cause problems; for instance, some versions of Solaris provide X Windows headers that expect __STDC__ to be either undefined or 1. See Invocation.


__STDC_VERSION__
This macro expands to the C Standard's version number, a long integer constant of the form yyyymmL where yyyy and mm are the year and month of the Standard version. This signifies which version of the C Standard the compiler conforms to. Like __STDC__, this is not necessarily accurate for the entire implementation, unless GNU CPP is being used with GCC.
The value 199409L signifies the 1989 C standard as amended in 1994, which is the current default; the value 199901L signifies the 1999 revision of the C standard. Support for the 1999 revision is not yet complete.

This macro is not defined if the -traditional-cpp option is used, nor when compiling C++ or Objective-C.


__STDC_HOSTED__
This macro is defined, with value 1, if the compiler's target is a hosted environment. A hosted environment has the complete facilities of the standard C library available.

__cplusplus
This macro is defined when the C++ compiler is in use. You can use __cplusplus to test whether a header is compiled by a C compiler or a C++ compiler. This macro is similar to __STDC_VERSION__, in that it expands to a version number. A fully conforming implementation of the 1998 C++ standard will define this macro to 199711L. The GNU C++ compiler is not yet fully conforming, so it uses 1 instead. We hope to complete our implementation in the near future.

__OBJC__
This macro is defined, with value 1, when the Objective-C compiler is in use. You can use __OBJC__ to test whether a header is compiled by a C compiler or a Objective-C compiler.

__ASSEMBLER__
This macro is defined with value 1 when preprocessing assembly language.

昨天写代码时需要在代码获取当前编译时间,从而可动态地作为版本信息,因此用到了C标准中的一些预定义的宏。在此将C标准中定义的几个宏一并总结一 下:

 

__DATE__ 进行预处理的日期(“Mmm dd yyyy”形式的字符串文字,如May 27 2006)
__FILE__ 代表当前源代码文件名的字符串文字 ,包含了详细路径,如G:/program/study/c+/test1.c
__LINE__ 代表当前源代码中的行号的整数常量
__TIME__ 源文件编译时间,格式微“hh:mm:ss”,如:09:11:10;
__func__ 当前所在函数名,在编译器的较高版本中支持
__FUNCTION__ 当前所在函数名

  对于__FILE__,__LINE__,__func__,__FUNCTION__ 这样的宏,在调试程序时是很有用的,因为你可以很容易的知道程序运行到了哪个文件的那一行,是哪个函数。
而对于__DATE__,__TIME__则可以获取编译时间,如如下代码通过宏获取编译时间,并通过sscanf()从中获取具体的年月日时分秒数据, 可在代码中做相应使用。我的代码中是根据此数据作为版本标识,并依此判断哪个版本新些及是否需要升级。
char * creationDate   = __DATE__ ", " __TIME__;
sscanf(creationDate, "%s %d %d, %d:%d:%d", month, &day, &year, &hour, &min, &sec);


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