C preprocessor

多次看到cpp这个工具，下面是应用举例：
cpp.exe -C -P -include macro_file infile outfile

参数说明：
Usage: cpp [switches] input output
Switches:
  -include <file>           Include the contents of <file> before other files
  -imacros <file>           Accept definition of marcos in <file>
  -iprefix <path>           Specify <path> as a prefix for next two options
  -iwithprefix <dir>        Add <dir> to the end of the system include paths
  -iwithprefixbefore <dir>  Add <dir> to the end of the main include paths
  -isystem <dir>            Add <dir> to the start of the system include paths
  -idirafter <dir>          Add <dir> to the end of the system include paths
  -I <dir>                  Add <dir> to the end of the main include paths
  -nostdinc                 Do not search the system include directories
  -nostdinc++               Do not search the system include directories for C++
  -o <file>                 Put output into <file>
  -pedantic                 Issue all warnings demanded by strict ANSI C
  -traditional              Follow K&R pre-processor behaviour
  -trigraphs                Support ANSI C trigraphs
  -lang-c                   Assume that the input sources are in C
  -lang-c89                 Assume that the input sources are in C89
  -lang-c++                 Assume that the input sources are in C++
  -lang-objc                Assume that the input sources are in ObjectiveC
  -lang-objc++              Assume that the input sources are in ObjectiveC++
  -lang-asm                 Assume that the input sources are in assembler
  -lang-chill               Assume that the input sources are in Chill
  -+                        Allow parsing of C++ style features
  -w                        Inhibit warning messages
  -Wtrigraphs               Warn if trigraphs are encountered
  -Wno-trigraphs            Do not warn about trigraphs
  -Wcomment{s}              Warn if one comment starts inside another
  -Wno-comment{s}           Do not warn about comments
  -Wtraditional             Warn if a macro argument is/would be turned into a string if -tradtional is specified
  -Wno-traditional          Do not warn about stringification
  -Wundef                   Warn if an undefined macro is used by #if
  -Wno-undef                Do not warn about testing udefined macros
  -Wimport                  Warn about the use of the #import directive
  -Wno-import               Do not warn about the use of #import
  -Werror                   Treat all warnings as errors
  -Wno-error                Do not treat warnings as errors
  -Wall                     Enable all preprocessor warnings
  -M                        Generate make dependencies
  -MM                       As -M, but ignore system header files
  -MD                       As -M, but put output in a .d file
  -MMD                      As -MD, but ignore system header files
  -MG                       Treat missing header file as generated files
  -g                        Include #define and #undef directives in the output
  -D<macro>                 Define a <macro> with string '1' as its value
  -D<macro>=<val>           Define a <macro> with <val> as its value
  -A<question> (<answer>)   Assert the <answer> to <question>
  -U<macro>                 Undefine <macro>
  -u or -undef              Do not predefine any macros
  -v                        Display the version number
  -H                        Print the name of header files as they are used
  -C                        Do not discard comments
  -dM                       Display a list of macro definitions active at end
  -dD                       Preserve macro definitions in output
  -dN                       As -dD except that only the names are preserved
  -dI                       Include #include directives in the output
  -ifoutput                 Describe skipped code blocks in output
  -P                        Do not generate #line directives
  -$                        Do not include '$' in identifiers
  -remap                    Remap file names when including files.

可以认为这是宏展开工具。个人认为比较有用的选项：
-dD        使用它，再配合BeyongCompare就可知道文件里，当前的宏被替换成了什么样子。也就间接知道了宏的定义情况。
-dM        比上面更方便，文件中用到的宏，全部输出成单独的定义文件，一目了然。

● 使用上面的工具是一次性打印出宏的定义情况。有的时候需要对“某一个宏”的存在、赋值情况进行精确的了解，那么就要用到GCC的pragma message：
#if defined(SOME_MACRO)
#    pragma message "[OK] the macro is defined"
#else
#    pragma message "[ERROR] no definition found"
#endif