【原创】Stringification 在二级宏定义中的使用

        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。