Preprocessor output

Question

How do I view the output produced by the C pre-processor, prior to its conversion into an object file?

I want to see what the MACRO definitions do t

Answer 1

You can check out my script described here:

http://mosermichael.github.io/cstuff/all/projects/2011/09/16/preprocessor.html

It formats the preprocessor output into a (hopefully) readable html document: lines that are different due to preprocessor are marked in the file.

Answer 2

It depends on the compiler you use.
With GCC, you can specify the -E flag on the command-line to let the compiler produce the pre-processor output.

Answer 3

You can also call the C Preprocessor directly.

cpp infile outfile

Check out man cpp for more info.

Answer 4

gcc -E file.c

or

g++ -E file.cpp

will do this for you. The -E switch forces the compiler to stop after the preprocessing phase, spitting all it’s got at the moment to standard output.

Note: Surely you must have some #include directives. The included files get preprocessed, too, so you might get lots of output.

For Visual C++ the switch is /E which spits the preprocessor output to screen.

Answer 5

For GCC,

gcc -E -dM file.c

or

g++ -E -dM file.cpp

should do the job. -dM, as GNU Preprocessor manual puts it, should generate a list of ‘#define’ directives for all the macros defined during the execution of the preprocessor, including predefined macros.