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