cpp (C preprocessor).

How to see macros expansion?

GCC:

$ cpp <file>.c

MSVC:

$ cl /E <file>.c

Under Emacs select region and type C-c C-e.

How to see predefined macros?

See:

http://predef.sourceforge.net/

predef project on Sourceforge

http://en.wikipedia.org/wiki/C_preprocessor#Compiler-specific_predefined_macros

Compiler specific predefined macros

http://msdn.microsoft.com/en-us/library/b0084kay.aspx

Predefined Macros

GNU C Compiler:

$ gcc -dM -E - < /dev/null

HP-UX ansi C compiler:

$ cc -v EMPTY.c

SCO OpenServer C compiler:

$ cc -## EMPTY.c

Sun Studio C/C++ compiler:

$ cc -## EMPTY.c

IBM AIX XL C/C++ compiler:

$ cc -qshowmacros -E EMPTY.c

For Visual Studio compiler there no any possibility get predefined macros from command line... But some macros documented:

_MSC_VER

Defines the compiler version. Always defined.

_WIN32

Defined for applications for Win32. Always defined.