-H

Prints to standard output, one per line, the path name of each file included during the current compilation. The display is indented so as to show which files are included by other files.

Here, the program sample.c includes the files, stdio.h and math.h; math.h includes the file, floatingpoint.h, which itself includes functions that use sys/ieeefp.h:

% cc -H sample.c    /usr/include/stdio.h
    /usr/include/math.h
        /usr/include/floatingpoint.h
             /usr/include/sys/ieeefp.h