本文转至【 https://my.oschina.net/moooofly/blog/126187 】
最近在研究 Redis-2.6.12 的源码时,重新对一处宏展开的用法进行了梳理,记录如下:
在 zmalloc.h 中有如下定义,
/* Double expansion needed for stringification of macro values. */ #define __xstr(s) __str(s) #define __str(s) #s
在注释中出现了一个专有名词“ stringification ”,经查阅,网络上比较认可的翻译是“字符串化”。所以上面双重宏定义的用途为“用于宏值字符串化的双重宏展开 ”。这个概念似乎比较模糊,无法让我深刻理解其用意,幸好有下面的 资料 :
3.4 Stringification Sometimes you may want to convert a macro argument into a string constant. Parameters are not replaced inside string constants, but you can use the ‘#’ preprocessing operator instead. When a macro parameter is used with a leading ‘#’, the preprocessor replaces it with the literal text of the actual argument, converted to a string constant. Unlike normal parameter replacement, the argument is not macro-expanded first. This is called stringification. There is no way to combine an argument with surrounding text and stringify it all together. Instead, you can write a series of adjacent string constants and stringified arguments. The preprocessor will replace the stringified arguments with string constants. The C compiler will then combine all the adjacent string constants into one long string. Here is an example of a macro definition that uses stringification: #define WARN_IF(EXP) \ do { if (EXP) \ fprintf (stderr, "Warning: " #EXP "\n"); } \ while (0) WARN_IF (x == 0); ==> do { if (x == 0) fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0); The argument for EXP is substituted once, as-is, into the if statement, and once, stringified, into the argument to fprintf. If x were a macro, it would be expanded in the if statement, but not in the string. The do and while (0) are a kludge to make it possible to write WARN_IF (arg);, which the resemblance of WARN_IF to a function would make C programmers want to do; see Swallowing the Semicolon. Stringification in C involves more than putting double-quote characters around the fragment. The preprocessor backslash-escapes the quotes surrounding embedded string constants, and all backslashes within string and character constants, in order to get a valid C string constant with the proper contents. Thus, stringifying p = "foo\n"; results in "p = \"foo\\n\";". However, backslashes that are not inside string or character constants are not duplicated: ‘\n’ by itself stringifies to "\n". All leading and trailing whitespace in text being stringified is ignored. Any sequence of whitespace in the middle of the text is converted to a single space in the stringified result. Comments are replaced by whitespace long before stringification happens, so they never appear in stringified text. There is no way to convert a macro argument into a character constant. If you want to stringify the result of expansion of a macro argument, you have to use two levels of macros. #define xstr(s) str(s) #define str(s) #s #define foo 4 str (foo) ==> "foo" xstr (foo) ==> xstr (4) ==> str (4) ==> "4" s is stringified when it is used in str, so it is not macro-expanded first. But s is an ordinary argument to xstr, so it is completely macro-expanded before xstr itself is expanded (see Argument Prescan). Therefore, by the time str gets to its argument, it has already been macro-expanded.
内容有点长,但确实是对 Stringification 作出了全面的解释。其中也对双重宏定义的用法进行了举例说明。针对 xstr(foo) 展开的情况,由于 xstr() 本身是一个符合普通宏展开定义的东东,而 foo 同样是这样的一个东东 ,所以,在对 xstr(foo) 进行宏展开的时,会按照正常的展开顺序进行,即先展开 foo,再展开 xstr 。
至于采用二级宏定义的好处,当然就是可以更加灵活的对宏展开时的内容进行控制。
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