Anyway to see list of preprocessor defined macros?

Question

I'd like to see all macros that are defined by the invocation of the compiler I'm using. Is there any way to do this? I have seen in the manual it says you can use cpp -dM but this doesn't work for me. Perhaps I'm doing something wrong?

When I run:

cpp -dM

I get no output at all from the preprocessor. If I try adding -dM as an option on gcc, I don't notice any difference.

Answer 1

You can use:

gcc -dM -E - < /dev/null

Note that you can also get the compiler macros in addition with this command:

touch bla.c && gcc -dM -E bla.c

For example on my computer:

$ touch bla.c && gcc -dM -E bla.c | wc -l
486
$ gcc -dM -E - < /dev/null | wc -l
124
$

Answer 2

By default, cpp -dM will read its input file from standard input and write to standard output. Since you're not trying to preprocess any input, you can pass it the empty input using /dev/null:

# Option 1
cpp -dM < /dev/null
# Optio n2
cpp -dM /dev/null

On Windows, you can use the NUL pseudofile instead of /dev/null.