g++: Is there a way to access compile flags inside the code that is being compiled?

Question

Is there a way (e.g., defined constants) to access compile flags with which the compiler was run inside the code that is being compiled.

For example, I want a program that writes the flags with which it was compiled.

int main(){
    std::cout << COMPILE_FLAGS << std::endl;
}

Do such constants exist for gcc/g++? Or even better: Are there constants that are defined both in gcc and clang?

I am especially interested in examining the optimization level and the value of the -march flag. So, if there are no constants that show all flags, are there at least ones that display these values?

Answer 1

The following command prints out all predefined macros:

g++ -dM -E - < /dev/null

This works with both gcc and g++. You can check yourself - unfortunately, there is no macro, that gives easy access to the full gcc/g++ command line.

Fortunately, most -m... flags result in adequate precompiler macros to be defined. For example, -m64 defines __x86_64 and -m32 defines __code_model_32__ . Or for -march: -march=core-avx2 results in #define __core_avx2__ 1 .

Just add the option, that you need to check, on the command line above, and check the result for new macro defines.