GCC dump preprocessor defines

Question

Is there a way for gcc/g++ to dump its preprocessor defines from the command line? I mean things like __GNUC__, __STDC__, and so on.

Answer 1

Late answer - I found the other answers useful - and wanted to add a bit extra.

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How do I dump preprocessor macros coming from a particular header file?

echo "#include " | gcc -E -dM -

or (thanks to @mymedia for the suggestion):

gcc -E -dM -include sys/socket.h - < /dev/null

In particular, I wanted to see what SOMAXCONN was defined to on my system. I know I could just open up the standard header file, but sometimes I have to search around a bit to find the header file locations. Instead I can just use this one-liner:

$ gcc -E -dM -include sys/socket.h - < /dev/null | grep SOMAXCONN
#define SOMAXCONN 128
$