can someone break this line apart gcc -E -dM - </dev/null

Question

Just came across this guy that left me stunned:

gcc -E -dM - </dev/null

This part is confusing to me:

 - </dev/null   

Answer 1

That gives a listing of all the predefined macros in gcc. -E means run the preprocessor. -dM means dump the predefined macros from the preprocessor. The - is for reading from standard input and /dev/null just feeds in an empty source file.