What c preprocessor macros have already been defined, gcc?

Question

In gcc, wow can I check what C preprocessor definitions are in place during the compilation of a C program, in particular what standard or platform-specific macrodefinitions are defined?

Answer 1

Predefined macros depend on the standard and the way the compiler implements it.

For GCC: http://gcc.gnu.org/onlinedocs/cpp/Predefined-Macros.html

For Microsoft Visual Studio 8: http://msdn.microsoft.com/en-us/library/b0084kay(VS.80).aspx

This Wikipedia page http://en.wikipedia.org/wiki/C_preprocessor#Compiler-specific_predefined_macros lists how to dump at some of the predefined macros

Answer 2

A program may define a macro at one point, remove that definition later, and then provide a different definition after that. Thus, at different points in the program, a macro may have different definitions, or have no definition at all.

Answer 3

A likely source of the predefined macros for a specific combination of compiler and platform is the Predef project at Sourceforge. They are attempting to maintain a catalog of all predefined macros in all C and C++ compilers on all platforms. In practice, they have coverage of a fair number of platforms for GCC, and a smattering of other compilers.

They achieved this through a combination of careful reading of documentation, as well as a shell script that figures out what macros are predefined the hard way: it tries them. My understanding is that it actually tries every string it can find in the executable image of the compiler and/or preprocessor to see if it has a predefined meaning.

They will happily add any info they don't have yet to their database.