B.2.104 -xM

Runs only the C preprocessor on the named C programs, requesting that the preprocessor generate makefile dependencies and send the result to the standard output (see make(1) for details about make files and dependencies).

For example:

#include <unistd.h>
void main(void)
{}

generates this output:

e.o: e.c
e.o: /usr/include/unistd.h
e.o: /usr/include/sys/types.h
e.o: /usr/include/sys/machtypes.h
e.o: /usr/include/sys/select.h
e.o: /usr/include/sys/time.h
e.o: /usr/include/sys/types.h
e.o: /usr/include/sys/time.h
e.o: /usr/include/sys/unistd.h

If you specify -xM and -xMF, the compiler appends all makefile dependency information to the file specified with -xMF.