[历史归档]本文原发布于 cstriker1407.info 个人博客内容为历史存档仅供参考。发布时间2017-05-26 标题【转】Stringification 在二级宏定义中的使用分类编程 / C C 标签宏·Stringification【转】Stringification 在二级宏定义中的使用本文转至【 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 3.4Stringification 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 onelongstring.Here is an example of a macro definition that uses stringification:#defineWARN_IF(EXP)\do{if(EXP)\fprintf(stderr,Warning: #EXP );}\while(0)WARN_IF(x0);do{if(x0)fprintf(stderr,Warning: x 0 );}while(0);The argumentforEXP is substituted once,as-is,into theifstatement,and once,stringified,into the argument to fprintf.If x were a macro,it would be expanded in theifstatement,but not in the string.Thedoandwhile(0)are a kludge to make it possible to writeWARN_IF(arg);,which the resemblance of WARN_IF to a function would make C programmers want todo;see Swallowing the Semicolon.Stringification in C involves more than puttingdouble-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 pfoo ;results inp \foo\ \;.However,backslashes that are not inside string or character constants are not duplicated:‘ ’ by itself stringifies to .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 whitespacelongbefore 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.#definexstr(s)str(s)#definestr(s)#s#definefoo4str(foo)fooxstr(foo)xstr(4)str(4)4s 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 isexpanded(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。